This is an emergency use case and not really practical but...
Line of sight is a major factor with GMRS so that being said, would it be feasible to have a helium balloon with a light weight repeater payload attached to a fishing rod and line to reel it back in?
You would have direct, unobstructed line of sight to the balloon repeater and it, being as high up as you have fishing line, would have line of sight extremely far.
Maybe in the kit have a one time use sized helium container for portability and to be a bit more practical? Obviously this isn't something you'd go out and do every day, but it would be an extremely useful kit to have in a vehicle that may get stranded without cell service or signal to connect to a repeater. Even just using simplex, this could be a life saver, right?