  In this work, we attempt to answer the question, "What is the minimal viable
renormalizable $SU(5)$ GUT with representations no higher than adjoints?". We
find that an $SU(5)$ model with a pair of vectorlike fermions
$5_F+\overline{5}_F$, as well as two copies of $15_H$ Higgs fields, is the
minimal candidate that accommodates for correct charged fermion and neutrino
masses and can also address the matter-antimatter asymmetry of the universe.
Our results show that the presented model is highly predictive and will be
fully tested by a combination of upcoming proton decay experiments, collider
searches, and low-energy experiments in search of flavor violations. Moreover,
we also entertain the possibility of adding a pair of vectorlike fermions
$10_F+\overline{10}_F$ or $15_F+\overline{15}_F$ (instead of a
$5_F+\overline{5}_F$). Our study reveals that the entire parameter space of
these two models, even with minimal particle content, cannot be fully probed
due to possible longer proton lifetime beyond the reach of Hyper-Kamiokande.
