Squall Lionheart said:
...isn't he the same as in FF8?
It kinda reminds me of my
"Returner" theory, so that each Gilgamesh in a Final Fantasy is the same person, but reincarnated in another world.
Anywho, this is what Wikipedia says about him:
FFIII:
"Gilgamesh is chancellor to Salonia's King Gorn. While controlling the king's mind, he forces Gorn to stab his own son Allus. Realizing what he's done, the king regains control. This angers Gilgamesh, causing him to summon the monster Garuda to fight the Light Warriors while he escapes. Gilgamesh is not seen again for the remainder of the game."
...probably died in an unlucky accident. ^^;
FFV:
"Gilgamesh is a lieutenant of Lord X-Death. He is big on talk, but prefers to put his own safety first; more often than not, he has others to do the fighting for him.
Gilgamesh is first seen on the alien planet, where he is told to guard the imprisoned Bartz, Reina and Faris. Galuf, the only one to escape captivity, infiltrates X-Death's castle and defeats Gilgamesh in a one-on-one battle.
Later the crew meet Gilgamesh again at the Big Bridge, through which the party of Bartz Klauser is trying to flee X-Death's captivity. Gilgamesh here puts up a real fight, but finds an excuse to flee after receiving enough damage.
Gilgamesh is next seen leading an attack on the naval fleet of Zezae Mathias Surgate. With the help of a demon called Enkido, he once again puts up an offensive, but is not as powerful as before.
Later in X-Death's castle, desperate to save face, Gilgamesh attacks Bartz's party with a sword he thinks is the legendary Excalibur, but which turns out actually to be a weak imitation called "Excalipoor".
Disgusted by Gilgamesh's perceived incompetence, X-Death promptly exiles Gilgamesh to the Cleft of Dimension.
When Bartz's party eventually reaches the Cleft of Dimension, they find Gilgamesh lost and frightened, reduced almost to tears. At first they prepare for combat, but then Gilgamesh soon ends the battle himself and takes off.
Gilgamesh's final appearance is again in the Cleft of Dimension, coming to the rescue of Bartz's party in their battle against a sealed monster called Necrophobia. Gilgamesh exchanges warm words of friendship with each of the party members, then self-destructs, annihilating both Necrophobia and himself in the process."
...You could say for the Returner theory, that the Cleft of Dimension is the gateway to other Final Fantasy worlds, where Gilgamesh journeys through directly. After passing away, the spirit of Gilgamesh of FFIII goes through this gateway, and reincarnates as Gilgamesh of FFV.
FFVIII:
"Gilgamesh is a randomly-visiting Guardian Force who replaces Odin in the fourth disc, if the player has already acquired Odin. Gilgamesh randomly chooses one of his four swords to attack all enemies on the screen: the mighty Masamune or Excalibur for heavy damage, the imitation Excalipoor for minimal damage (one hit point), or Odin's Zantetsuken for an instant kill.
According to the story, this is supposed to be a crossover from Final Fantasy V, where Gilgamesh would have escaped from the Cleft of Dimension.
He actually first appears in the final battle against Seifer in Lunatic Pandora, which opens with Odin being killed by Seifer himself. If the battle lasts about 10-15 rounds, Gilgamesh will appear, and (in a war of words) he defeats Seifer, his final remark being a reference to Final Fantasy V."
...So it's pretty certain that the FFV Gilgamesh and FFVIII Gilgamesh are the one and the same.^^ By the powerful self-destruction, killing the foe Necrophobia, he somehow transforms in a powerful Guardian in the Cleft of Dimension itself.
FFIX:
Gilgamesh is a self-proclaimed great treasure hunter who refuses to tell anyone his name. The player encounters this four-armed man multiple times during his journey. The final location where he stays until the end of the game is at the Daguerreo Library. Unlike in previous Final Fantasy games, this Gilgamesh is not associated with battling. Instead, he accredits Zidane, the main character, a rank based upon the rate of stealing items in battle throughout the game. Upon reaching rank S, Gilgamesh gives the key item Rank S Medal in recognition of the feat, and it is then that he confesses his name as well as the lies about him being a great treasure hunter, just before leaving never to be seen again.
Also if you receive the Excalibur II, there is a small note addressed to Gilgamesh about the Excalibur II and Excalipoor:
"To Brother Gil
Bro, I found the sword, like you told me. But there were two. One of 'em had a lame name, Something II. It was a dingy, old thing with flashy decorations, something you'd probably like. So I went with Excalipoor. I'll be back after I find the Tin Armor." "
... Since the Cleft of Dimension could be a gateway/world of spirits, it could be that the Guardian Force powers Gilgamesh obtained slowly vanished, and reïncarnated as 4-armed Human on Gaia. Looking at this note, the Cleft of Dimension could be linked to the Crystal World/thus Memoria, thus leading to Gaia. He probably ended up first in Memoria as a Human, since Excalibur II and the Tin Armor are two items you can achieve from Hades, in Memoria. ^^;

So yeah, i made up for my self: Gilgamesh from FFIII, FFV, FFVIII and FFIX are the one and the same.

^_^...