<p>I'll use the F/GO game (English version) for the basis of my answer. I use a somewhat humorous name in-game, so try not to mind that too much.  There are definitely differences with the animation (there's no persuading anything about a nega-genesis whatsit), so it's possible there are differences between them regarding how any given character arrives on a given scene. But supposing we put that aside...The short of the answer is: we don't know how Gilgamesh himself arrived there in Archer form. The longer answer is...</p>
<p>First, (real) Merlin is not the one who summons First Hassan. While Merlin describes First Hassan as being an Ace he's kept hidden up his sleeve, he denies being the one to actually summon him.  Nor was he summoned by Gilgamesh or a (greater) grail.  First Hassan is in fact summoned by the protagonist, albeit without his knowing it.</p>
<p><a href="https://i.sstatic.net/Kr5Hh.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Kr5Hh.jpg" alt="Real Merlin rhetorically asks who summoned First Hassan" /></a>
<a href="https://i.sstatic.net/Sr9Vs.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Sr9Vs.jpg" alt="It was the PC all along" /></a>
As I suggested in my comment, this kind of implies that First Hassan kind of summoned himself there so he could fight a Beast, as is the point and calling of a Grand Servant, and to be a Servant to the protagonist.</p>
<p>The real Merlin has arrived by typical Merlin rules-lawyering shenanigans.
<a href="https://i.sstatic.net/2gPkd.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2gPkd.jpg" alt="Merlin abuses another loophole" /></a></p>
<p>Unfortunately, Gilgamesh is even less forthcoming with how he managed to arrive as an Archer class Servant.
<a href="https://i.sstatic.net/55JxS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/55JxS.png" alt="Gilgamesh simply says that breaking rules is easy for a King" /></a>
When Mash asks/implies that he survived to this point after all, he responds:
<a href="https://i.sstatic.net/57POr.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/57POr.png" alt="Sort of.  There's room for compromise in the Underworld." /></a>
And that's all we hear on the matter.  So he broke and bent the rules, using the nature and mechanics of the (Babylonian) Underworld to his advantage.  He does seem to have full memory of his association with the protagonist and Mash.  Nevertheless, it's unclear what has been accomplished here.  He calls himself a Servant, and I think we can trust him on that, so we can rule out that he's simply the human Gilgamesh the story had featured thus far donning a new role.  Perhaps he is a Demi-Servant or Pseudo-Servant, using his own body and/or spirit as the base?  Or, heck, maybe somewhere in that vault he's got a &quot;turn a human/spirit into a Servant&quot; potion or another, and it really is the human (caster) Gilgamesh (or at least his spirit).  It's impossible to say from what we're provided, unfortunately, beyond that it relies in part on some sort of shenanigans involving the rules and nature of the Underworld, and that he is in fact a Servant of some sort.</p>

CC BY-SA 4.0
2021-03-11T12:03:09.080Z
Thank you @zibadawa timmy, Gilgamesh breaking the rules of the Babylonian underworld as Eresh was busy fighting Tiamat seems like the best guess to me as well. So Gilgamesh' unbound spirit used his physical body as catalyst to summon himself in the underworld. Though I wonder if Gilgamesh (Archer servant) then had to disappear and abandon his rule over Babylonia after defeating Tiamat as there was no master who could supply him with mana anymore.
Erik Vollstaedt
CC BY-SA 4.0
2021-03-10T09:58:37.520Z
2021-03-10T10:07:03.620Z
2021-03-10T10:07:03.620Z
zibadawa timmy
<p>When Tiamat attempted to snipe Ritsuka with a long range attack, Gilgamesh blocked it and died to save Ritsuka. Kingu then sacrifices himself to restrain Tiamat with the Chain of Heaven long enough for Ereshkigal to finish moving the Underworld under Tiamat. Ishtar blasts a hole underneath Tiamat, causing her to fall into the Underworld. Later, when Tiamat summons high class Lahmu to fight Ritsuka and his servants while she climbs out of the underworld, Gilgamesh arrives in his servant form to support Ritsuka's team against Tiamat and the Lahmu and uses Enuma Elish later to finish off Tiamat's physical form after Ritsuka persuaded her to give up her Nega-Genesis field to recreate the universe. How was Gilgamesh summoned? I know the real Merlin arrived from Avalon and summoned the Grand Assassin Hassan, but there was no mention of him summoning Gilgamesh in his Archer form as well.</p>
<p><a href="https://i.sstatic.net/wBmNx.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/wBmNx.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-01-25T20:15:04.093Z
In the game at least, Hassan is there long before Tiamat emerges, and he is not in the Grand class; he's in a regular servant level body.  He's masquerading as an old man in the city and tries to give the MC a lesson in manners (this is all ultimately why one of his nicknames is "Gramps").  It's implied he sort of manifested himself there just so he could have the pleasure and honor of joining the MC's fight, willfully ditching the Grand class for it.  Both Merlin's second appearance and Archer Gil's appearance were waved off as underworld loophole abuses, I think, but not sure.
zibadawa timmy
CC BY-SA 4.0
2021-01-25T20:38:52.047Z
I remember in the series that Hassan also appeared before, both in Uruk as well as in the underworld: 

Initially taking the appearance of an old, bearded vagrant and assuming the name of Ziusudra, he awaits the protagonist in the city of Uruk—apparently to again test the protagonist's character. Subsequently, during the protagonists' exploratory mission to the abandoned city of Cuthah, he intervenes when the protagonist's soul is drawn into the Underworld by Ereshkigal.  

https://typemoon.fandom.com/wiki/First_Hassan#Babylonia:_The_Absolute_Frontline_in_the_War_Against_the_Demonic_Beasts
Erik Vollstaedt
CC BY-SA 4.0
2021-03-09T16:56:02.990Z
There are plenty of theories in this reddit thread, though I'm still not entirely sure who is right.. https://www.reddit.com/r/grandorder/comments/fvsnu9/babylonia_how_was_king_gilgamesh_able_to_summon/
Erik Vollstaedt
<fate-grand-order>
How is Gilgamesh' summoned at the final battle against Tiamat in the Underworld in Fate/Grand Order - Absolute Demonic Front: Babylonia?
# How is Gilgamesh' summoned at the final battle against Tiamat in the Underworld in Fate/Grand Order - Absolute Demonic Front: Babylonia?
Posted by: username_0 on 2021-01-25
When Tiamat attempted to snipe Ritsuka with a long range attack, Gilgamesh blocked it and died to save Ritsuka. Kingu then sacrifices himself to restrain Tiamat with the Chain of Heaven long enough for Ereshkigal to finish moving the Underworld under Tiamat. Ishtar blasts a hole underneath Tiamat, causing her to fall into the Underworld. Later, when Tiamat summons high class Lahmu to fight Ritsuka and his servants while she climbs out of the underworld, Gilgamesh arrives in his servant form to support Ritsuka's team against Tiamat and the Lahmu and uses Enuma Elish later to finish off Tiamat's physical form after Ritsuka persuaded her to give up her Nega-Genesis field to recreate the universe. How was Gilgamesh summoned? I know the real Merlin arrived from Avalon and summoned the Grand Assassin Hassan, but there was no mention of him summoning Gilgamesh in his Archer form as well.

[![enter image description here](https://i.sstatic.net/wBmNx.jpg)](https://i.sstatic.net/wBmNx.jpg)
**Tags:** fate-grand-order
> **Comments:**
> **username_1**: In the game at least, Hassan is there long before Tiamat emerges, and he is not in the Grand class; he's in a regular servant level body.  He's masquerading as an old man in the city and tries to give the MC a lesson in manners (this is all ultimately why one of his nicknames is "Gramps").  It's implied he sort of manifested himself there just so he could have the pleasure and honor of joining the MC's fight, willfully ditching the Grand class for it.  Both Merlin's second appearance and Archer Gil's appearance were waved off as underworld loophole abuses, I think, but not sure.  
> **username_0**: I remember in the series that Hassan also appeared before, both in Uruk as well as in the underworld: 

Initially taking the appearance of an old, bearded vagrant and assuming the name of Ziusudra, he awaits the protagonist in the city of Uruk—apparently to again test the protagonist's character. Subsequently, during the protagonists' exploratory mission to the abandoned city of Cuthah, he intervenes when the protagonist's soul is drawn into the Underworld by Ereshkigal.  

https://typemoon.fandom.com/wiki/First_Hassan#Babylonia:_The_Absolute_Frontline_in_the_War_Against_the_Demonic_Beasts  
> **username_0**: There are plenty of theories in this reddit thread, though I'm still not entirely sure who is right.. https://www.reddit.com/r/grandorder/comments/fvsnu9/babylonia_how_was_king_gilgamesh_able_to_summon/  
---
## 1 answers
### Answer by username_1 on 2021-03-10. Score: 1 | Accepted answer
I'll use the F/GO game (English version) for the basis of my answer. I use a somewhat humorous name in-game, so try not to mind that too much. There are definitely differences with the animation (there's no persuading anything about a nega-genesis whatsit), so it's possible there are differences between them regarding how any given character arrives on a given scene. But supposing we put that aside...The short of the answer is: we don't know how Gilgamesh himself arrived there in Archer form. The longer answer is...

First, (real) Merlin is not the one who summons First Hassan. While Merlin describes First Hassan as being an Ace he's kept hidden up his sleeve, he denies being the one to actually summon him. Nor was he summoned by Gilgamesh or a (greater) grail. First Hassan is in fact summoned by the protagonist, albeit without his knowing it.

[![Real Merlin rhetorically asks who summoned First Hassan](https://i.sstatic.net/Kr5Hh.jpg)](https://i.sstatic.net/Kr5Hh.jpg)
[![It was the PC all along](https://i.sstatic.net/Sr9Vs.jpg)](https://i.sstatic.net/Sr9Vs.jpg)
As I suggested in my comment, this kind of implies that First Hassan kind of summoned himself there so he could fight a Beast, as is the point and calling of a Grand Servant, and to be a Servant to the protagonist.

The real Merlin has arrived by typical Merlin rules-lawyering shenanigans.
[![Merlin abuses another loophole](https://i.sstatic.net/2gPkd.jpg)](https://i.sstatic.net/2gPkd.jpg)

Unfortunately, Gilgamesh is even less forthcoming with how he managed to arrive as an Archer class Servant.
[![Gilgamesh simply says that breaking rules is easy for a King](https://i.sstatic.net/55JxS.png)](https://i.sstatic.net/55JxS.png)
When Mash asks/implies that he survived to this point after all, he responds:
[![Sort of.  There's room for compromise in the Underworld.](https://i.sstatic.net/57POr.png)](https://i.sstatic.net/57POr.png)
And that's all we hear on the matter. So he broke and bent the rules, using the nature and mechanics of the (Babylonian) Underworld to his advantage. He does seem to have full memory of his association with the protagonist and Mash. Nevertheless, it's unclear what has been accomplished here. He calls himself a Servant, and I think we can trust him on that, so we can rule out that he's simply the human Gilgamesh the story had featured thus far donning a new role. Perhaps he is a Demi-Servant or Pseudo-Servant, using his own body and/or spirit as the base? Or, heck, maybe somewhere in that vault he's got a "turn a human/spirit into a Servant" potion or another, and it really is the human (caster) Gilgamesh (or at least his spirit). It's impossible to say from what we're provided, unfortunately, beyond that it relies in part on some sort of shenanigans involving the rules and nature of the Underworld, and that he is in fact a Servant of some sort.
> **Comments:**
> **username_0** (2021-03-11): Thank you @zibadawa timmy, Gilgamesh breaking the rules of the Babylonian underworld as Eresh was busy fighting Tiamat seems like the best guess to me as well. So Gilgamesh' unbound spirit used his physical body as catalyst to summon himself in the underworld. Though I wonder if Gilgamesh (Archer servant) then had to disappear and abandon his rule over Babylonia after defeating Tiamat as there was no master who could supply him with mana anymore.  
---
