I have leftover baked chicken breast. I want to know if I could reheat it on my gas grill and if it would still taste good after that.
You can, but you'll likely end up drying it out if you just slap it on the grill.
If you're just trying to warm it up while grilling something else, I'd put it in a (relatively flat) foil packet, and use low or indirect heat, so you're just warming it, not heating it so far that you overcook it.
Joe's excellent suggestion will keep it from drying out, but if you want to gain some benefit from the flavor that grilling can add to your chicken, try removing it from the foil packet after it's thoroughly heated, brush on some BBQ sauce and let it grill for a couple of minutes (directly on the grill) just until the sauce starts to caramelize.