I hammocked at Husky Haven campground and inside the Connellsville shelters while on the GAP. I had called Husky before going and the woman there told me which site to take for hammocking. She had hammockers there before me. At the shelters it was only my co-rider and me in one of the two shelters - the end of May before all the rush.
Roundbottom Primitive Campground & Cedar Creek Park Campgrounds are good for hammocking.
I plan on hammocking again on the GAP this month.