[project935]

Everything should be galvanized steel - including the fenders (unless someone replaced them with fiberglass). Gage (thickness) will often vary from structural parts (rails, rockers, wheel houses, etc) to cosmetic panels (fenders, door skins, etc).
If replacement parts can't be used (new or used), a general guideline is to make patch panels from 20-22 gage for cosmetic panels and 18-20 gage for the structural parts - depends on the size and complexity of the area to be patched.