dont invest in Dmax Gengar
many people getting ready for Gmax gengar are saying to get Dmax genger ready as an attacker for it, and yes, Dmax gengar will be one of the best attackers to hit gmax gengar with, especialy with stab ghost max attack... but then it will be imediatly outclassed after one win.
instead, invest in Metagross, its attack stat is pretty much the same as gengar's and it also has stab super-effective psychic attacks. but metagross doesnt have a Gmax version and will stay relevant for almost every other Gmax raid as a tank or attacker or healer all the way up until Eternatus where meragross is a perfect counter!
plus, dmax beldums are available again and very easy to do solo now with a cinderace or charizard. so dont waste stardust and MP and candy on your Dmax gengars! save your resources
Edit: im not saying to use Metagross to tank for Gmax gengar, just for damage, instead of leveling a dmax gengar just for the gmax fight (well unless the gmax gengar uses poison moves, then metagross can tank). but for ghost attack gmax gengars use greedent or dubwool to tank, im just saying dont level a dmax gengar just to use for max attacking gmax gengar.