obi
formerly david stone
I don't like the inaccurate move Hypnosis on Gengar, due to its frailty. However, if I had some sort of assurance that Blissey didn't have Thunder Wave, then I could see it being useful to let you actually kill it (Focus Punch on the switch, spam Hypnosis until it hits or Blissey switches). I agree that Substitute isn't the best general choice, but that's more because I like the idea of Life Orb. Of course, with how much stuff Gengar hits SE, Expert Belt may just be the better idea.
Another idea that I never really got around to using outside of ADV ubers is Light Screen + Subpunch Gengar. Sure, Blissey might break your Substitute with Ice Beam without fail, but throw up a Light Screen, and suddenly Blissey needs a critical hit to get through. Light Screen also means that SpecsMence really can't come in and terrorize you until it wears off, or Hippowdon can just stay in and use Ice Fang. Of course, I don't know that I'd use Hippowdon with a SubGar, but it's just an example.
Another idea that I never really got around to using outside of ADV ubers is Light Screen + Subpunch Gengar. Sure, Blissey might break your Substitute with Ice Beam without fail, but throw up a Light Screen, and suddenly Blissey needs a critical hit to get through. Light Screen also means that SpecsMence really can't come in and terrorize you until it wears off, or Hippowdon can just stay in and use Ice Fang. Of course, I don't know that I'd use Hippowdon with a SubGar, but it's just an example.