Eh, I'm not surprised Gackt ended up being in some FF game or another. Afterall, Squall Lionheart was based off Gackt, appearance wise, and Vaan from XII, is a cross between Gackt and this other J Rock singer called Hyde. Besides, I like Gackt as an actor, and I really like the way they included him in the game. He does all the movements and all himself, and they just record him doing that, and then lay CG over and around him. Very clever. It's good marketing for Square Enix, also.

I mean, they got Gackt in one of their games for crying out loud! xD
But, I have no idea what he his role in the game is.
My theory is that he DOES have some relation to Sephiroth. They both have one wing, and Gackt's is on the opposite shoulder. Also, apparently he only appears in some secret ending, and doesn't actually have that big a role in the game.
I have yet to play and complete the game myself, so... I could be wrong.