It’s not good in gen 8 AG at all.
It’s outclassed in every single way by the king of the tier, Calyrex-Shadow rider. Calyrex-Shadow has every single stat better than Gengar’s. Calyrex-Shadow happens to also be a ghost type, and it completely overshadows Gengar. There is absolutely no reason to use it over Calyrex-Shadow, or other awesome Special Sweepers in the tier like Kyogre and Eternatus.
If you want to use Gengar, the best place to do it is Monotype, as that is the tier where it isn’t completely outclassed. It’s got very good Special Attack and Speed, good STABs in Sludge Wave and Shadow Ball, and good coverage moves like Giga Drain, Thunderbolt, and Dazzling Gleam.