We stayed at both campgrounds last year. And have stayed at Gros Ventre several times previously.
Gros Ventre does have some sites close together or backing each other and some farther apart. We love that place. I feel when they relaid the concrete pads a few years ago, they could have/should have made some longer. The dump station rules were implemented a few years ago because the river was showing signs of pollution. Many folks not actually staying there would pop in to use it.
Colter Bay, as Larry states has an RV park with hook ups (not sure if full) in the trees which are very close together. We drove around it and weren't keen. The campground sites have parallel spaces and are actually fairly spacious, although you are parked closely together, especially if bigger rig, you have a lot of room in your spot behind rig.
I think the roads need work. Too many potholes. Other than that I like the rustic feel and would hate it to become busier than it already is.