Give him a shot and let him slide to the AHL if he doesn't impress in preseason+9 game stint. It's really that easy. I know a lot of people are mad the offense isn't there, but hockey players dont' come in one mold. You can't just try to make a team of Ron Francis and be mad when you lose.
Given his size, I'd rather he go to the minors and adjust. Its true for the hulking behemoths of the juniors as well as the undersized guys that you need to adjust, having guys that are taller and stronger means you won't be able to get away with much so you might as well get some seasoning in, not to mention the AHL squad can give him some good minutes. They'll probably end up running Nylander, Froese, Gauthier down the middle.
I think Gauthier will be one of the Leaf prospects that hangs around the team. Maybe they see him as a Hanzal or Maholtra type player, and that's great. I could see alot more of these prospects get shipped out in a package to bring in a quality top line talent instead.