A natural gas fire grill MIGHT be considered similar to a gas fired stove. Some are rated for flammable surface contact. But, they tend to produce a LOT of smoke, depending on what you're cooking, so that could easily be problematic. I've never looked into this, but you probably will have issues both in operation and code. Somebody may know, so don't give up hope yet. At the least, you'd want a really good exhaust fan to clear the smoke from the garage. Some of the smoke will contain grease, and that will deposit on surfaces, making for a potential fire hazard if you don't install a good hood.