Most awnings will come with gutter mount clips, these are pretty good although you will have a gap between vehicle and awning so can get cold, windy and wet by the door. You can also get suction mounts for the roof, cheap ones are crap but good ones are very strong although pricey. Awning rail is your best bet, screws straight to the roof and can be fitted inside the gutter if wanted to, this provides the best seal for an awning and is what I use on camper vans, keeps heat in, drafts and water out...