#2 Potential gimmick
If you want to bring GSP in as a good guy, then his gimmick really is quite simple - have him use his old UFC attire, smile and wave to the crowd and just focus on whichever opponent is put in front of him. If you give St-Pierre a nice persona, then he'll be seen as a man of split personalities once you put him in the ring and he shows the world how talented he really is.
On the opposite end of the spectrum, a Terminator-esque gimmick as a heel could work wonders for him. It'd be similar to the character he played in Captain America, however, we literally wouldn't have him say a single word. Just go out there, look into the soul of the other wrestler with those steel blue eyes and prove why you're such an animal.
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Now to the most obvious point of all.