Hell’s Gate SP is one of our favorites. We have stayed there and done day trips south south over WA-129/OR-3 down the Grande Ronde Gorge to Enterprise / St. Joseph / Imnaha and the Eagle Cap Wilderness in the Wallowas. Extremely spectacular. I have no recommendations for camping in that area south of the Grande Ronde.
Driving along the Snake River Gorge south of Asotin, WA is very cool, but you cannot get into Hell’s Canyon on that stretch. Take a jet boat out of Lewiston.
Away from Lewiston it gets pretty remote.