So, in the original Epic, Gil was a horrible tyrant who abused his people like disposable, and sexual, livestock. Enkidu kind of smacked some sense into Gil because Enkidu was the first entity that was at the same power level as Gil. The tyranny became far less after that because Gil had discovered the wonders of Bromance.
Fast Forward, Enkidu dies from a curse of the Gods (another reason Gilgamesh isn’t really keen on them aside from his existence being the Gods way of keeping tabs on humans) and Gilgamesh realizes that he’s gonna die one day as well, no matter how powerful he is. So Gil goes on a Journey (that word is important) to obtain immortality. After he achieves this pretty dang impossible task, the herb that gives it was eaten by a snake which caused it to be able to renew its existence by shedding it’s skin. This is the snake skin used as a catalyst to Summon Gil in Fate Zero, and makes a cameo in Stay Night.
Instead of being upset about losing his chance at immortality, or going back to grab another herb, which would have been doable for Mr OP. Gil just kinda chills and accepts that he has limitations and to live within them. After returning from his Journey (Post Journey), Gil is pretty relaxed, has the wisdom to see through all matters, and basically becomes a good ruler… who will still murder your ass if you’re impertinent, but that’s just a King thing, so…
The Gilgamesh in both Zero and Stay Night is Pre-Journey Gilgamesh, a selfish arse who kills whenever his mercurial nature sways that way and easily bored by the crap happening around him. A full out despotic tyrant. So… why was Pre-Journey Gil summoned from a relic that was from the Post Journey phase of Gil’s life? I personally think it’s because Tohsaka Tokiomi’s influence as a master caused that part of Gil to manifest. Lots of pride, ruthless, willing to kill family friends for his own ambition. So, the Master has a lot of influence on the aspect of the Servant being summoned.
Now, the Gilgamesh in Zero is Pre-Journey. Raging D-Bag. So much so that even after spending 10 years in flesh and blood incarnation in the human world from his contact with the Grail, he’s still a raging D-Bag in Stay Night. He’s set his sights on making Artoria his Queen, or at the very least, his regular Saturday Night Thing, with or without her consent. This comes to a head in the Stay Night route where Gilgamesh gets so fed up with his failed courtship attempts that he basically pulls a Mike Tyson and tells Artoria “I will rape you until you love me.”
The two warrior kings then go at each other and, no surprise, because storytelling, Artoria triumphs. Now, in the final moment before he disappears, Gilgamesh reaches out to Artoria and with resignation says, “There are things in this world that cannot be collected after all, which makes them all the more precious.” This is the moment of epiphany that happened to Post-Journey Gil when the Herb was eaten by a snake. So at this moment, Pre-J Gil in the Fate Series becomes Post-J, and only at the moment of his death.
Now, Fate/ Grand Order happens, and all the original cast becomes available for Summoning. I gave as good an explanation for that in my Fan Fic as possible. And Gil is also Summoned. In the FGO universe, I theorize it is the Stay Night Gil from the moment after he became Post-J Gil, so he’s learned his life lesson and is a lot more reasonable, even though he’s the same rapacious Arse he was before. He’s just a bit less murderous and slightly most snuggly. Which is why he’s able to remain in Chaldea without too many problems. He has chilled, and is on the path to becoming the Wise King that CasGil is.
The reason I wrote this thesis is because there is a part of the Reddit Board that exploded in debate over Gilgamesh and his character. So, thank you all for reading and I hope this has been helpful.