Filling gap between bottom of tiles and shower pan?
Just as a matter of terminology, grout is not used to adhere tiles.  Thin-set mortar is used to adhere tiles to floors and walls, and mastic adhesive is used to adhere tiles to walls.  Grout is similar to Portland cement and goes between the tiles.  I only bring this up because if you go to a hardware store and ask for grout, they'll probably give you grout, which is not what you want.

Your best bet would probably be to use epoxy thin-set or epoxy bonding mortar.  Being an epoxy, it should stick much better to the fiberglass and is suitable for use as a mortar.  Personally, I'd use it to adhere some cement board in that gap (pending a description of how wide the gap is) and then the rest to adhere the tile to the cement board.  Let it set, and then you can grout.