As far as special ghost sweepers, it’s amazing. I bring it to Gen 8 Ubers and it can really get work done. If you want a physical ghost though, I like Marshadow the best, as it can use Technician well. Dragapult is also both a physical and special sweeper, and I’ll talk about it later. Still, Gengar is one of the better special sweepers out there. It has a lot of coverage, such as its Giga Drain+ Big Root combo, though Black Sludge is also a good option. My Gengar looks like this:
Gengar @ Black Sludge
Ability: Cursed Body
EVs: 252 SpAtk, 252 Spe, 4 SpDef
IVs: 0 Atk
I haven’t used a lot of ghosts, but I can tell you that Gengar is one of the better ones. It can hit hard while healing with Black Sludge. I usually use physical sweepers, and this is one of the few special ones I have used. I highly recommend it if you want to use Gengar, but as far as special ghosts though, there is a better one in Gen 8.
Special Dragapult. It has incredible speed, outspeeding Gengar. It runs a Choice Specs quite commonly due to it having access to U-Turn. I’d say a good move set would be U-Turn, Shadow Ball, Draco Meteor, and Fire Blast with Choice Specs, max speed and special attack, with Timid/Modest. I’d say this is the the best special ghost in the games, though this is all just my opinion from studying both Ghosts, you don’t have to agree with me, but I still think Dragapult is the best.
Hope this helps!!! :)