Could we create a Hypnosis TM? There’s one for Toxic, Will-O-Wisp, and Thunder Wave, and there’s no move that just freezes (unless that’s also planned) so the only sleeping-causing TM is Rest, which doesn’t help a Nightmare/Dream Eater Gengar set.
Gastly, Gengar etc, learn/know hypnosis at level 1. If yours doesn’t know it, you can always reteach the move.