  GSN 069 is the first galactic nucleus where quasi-periodic eruptions (QPEs)
have been identified. These are high-amplitude, soft X-ray bursts recurring
every ~9 hr, lasting ~1 hr, and during which the X-ray count rate increases by
up to two orders of magnitude with respect to an otherwise stable quiescent
level. The X-ray spectral properties and the long-term evolution of GSN 069 in
the first few years are consistent with a long-lived tidal disruption event
(TDE). Here we derive the properties of QPEs and of the long-term X-ray
evolution in GSN 069 over the past 12 yr by studying timing and spectral X-ray
data from 11 XMM-Newton, one Chandra, and 34 Swift observations on timescales
ranging from minutes to years. QPEs in GSN 069 are a transient phenomenon with
a lifetime > 1.05 yr. The QPE intensity and recurrence time oscillate and allow
for alternating strong-weak QPEs and long-short recurrence times to be defined.
In observations with QPEs, the quiescent level exhibits a quasi-periodic
oscillation with a period equal to the average separation between consecutive
QPEs. The QPE spectral evolution is consistent with thermal emission from a
very compact region that heats up quickly and subsequently cools down via X-ray
emission while expanding by a factor of ~3 in radius. The long-term evolution
of the quiescent level is characterised by two repeating TDEs ~9 yr apart. We
detect a precursor X-ray flare prior to the second TDE that may be associated
with the circularisation phase during disc formation. A similar precursor flare
is tentatively detected just before the first TDE. Future X-ray observations of
GSN 069 promise that the QPE origin and the relation between QPEs and repeating
TDEs in this galactic nucleus will be constrained, with consequences for the
other sources where QPEs have been identified. [abridged]
