I never made it to endgame content (if there is any) on FFXI so I don't know if they implemented him in any way in FFXI. I mean considering how darn funny and how long he has been a good Villain, cameo, and all around clown. He would be amazing as some sort of endgame boss. maybe even a cut-scene for when he goes into his 8 armed form. And once defeated he would drop a legendary weapon he possesses. (which would cover Gld, Lnc , and Mar in terms of gearing) And considering the Rift is well inter-dimensional (Hence him appearing in all the others besides V) There is no real way he could be integrated and it not completely ruin the general storyline of the game.
Just a bit of a suggestion...
But seriously. We gotta have Gilgamesh...
Edit: And please for the love of the twelve..make the fight on a big bridge... |: