In RBY Gengar is in no way outclassed by Psychic-types. It serves a unique niche as fastest sleeper, Explosion absorber, and that thing that forces Tauros and Snorlax to run Earthquake (well, okay, Counter Chansey plays a role in that regard, too).
In GSC Misdreavus is actually the premier spin blocker, Gengar is more prized for his offensive versatility.
In ADV Gengar is the bomb, unlike Dusclops it can actually kill shit, so it's usually the preferred spinblocker nowadays. The introduction of EVs and that new Ground immunity to abuse made Special Sweeping not much of a problem, even without STAB, as long as Blissey/Snorlax weren't around, and even then it had ways around those with Sub+Focus Punch, Taunt + WoW, Dbond, and Explosion.
I don't know much about DPP but I think Gengar manages to shore up most of its issues with those immunities and that fast Substitute. Plus it still has a very versatile movepool as always, sure it no longer has SE coverage against literally everything but it still gets Pain Split, Explosion, Focus Punch, and Hypnosis. Gengar has never been solely about attacking prowess (how could it be without a STAB this whole time), but rather all the other toys it gets to wreck special walls, too.
Gengar is not problematic at #4 at all.