As reflected by its position in the VR, Gengar is a viable, but incredibly niche special attacker that abuses the lack of Ghost resists in the tier. The tiers main forms of Ghost management—Tyranitar and Chansey—are dealt by Focus Blast and Taunt. Gengar’s ability to deal with the former as well as its higher Speed tier and Willo/Hex separate it from the tier’s other Ghost-type, Blacephalon. Not all is good for Gengar, though. It is very frail, and brings zero defensive utility to the team barring Normal resist, Fighting resist and ability to absorb Toxic Spikes (the former two don’t really matter when the strongest mons of those types—Mega Medicham and Mega Lopunny—have other ways of managing it). Also, remember Blacephalon? It’s stronger, actually fully capable of breaking non-Kommo-o stalls with SubCM, can boost its Speed whilst maintaining wallbreaking prowess, and can cripple its checks with the correct lure (HP Grass for Ttar and Trick/Knock for Chansey/Mantine). Overall, I’d recommend using Blacephalon, but be my guest if you want to bring out Gengar’s qualities.