*Didn't see Willdbeast had posted until now*Normalize Gengar is now one of those things which you have to make at least one counter or your team dies. I wish it wasn't the case. I feel like it's something that should be banned soon. Ah well. Here's some counters for it.
The first idea to counter it is only an idea and not a set. It's Magic Bounce. This is one of the better ideas since most carry Entrainment and not Skill Swap. The idea is to swap in the MB user against the Gengar and have it waste a turn. I'd recommend putting Nuzzle on it to slow it down so your heavy hitter can swap in and kill it like that. It doesn't work against Skill Swap Gengars though.
Judgement is also a good idea, for it is only affected by the plate you're holding, allowing for a powerful Ground, Psychic, Dark, or Ghost attack. Multi-Attack is worse but works on a physical set whilst Judgement is better on a special attacker.
My final idea is countering with an Entrainment and Prankster Pokemon. Prankster allows for you to go first so you can change it's ability and force a switch, and with Pursuit you can deal heavy damage to it when it does.
As my prior post states, a bug for Arceus let’s it choose a form (like Ghost) and have a Revelation Dance for that type, without actually being the form (no plate or multi-type ability).
I would also suggest is Priority. Think of Choice Band Tough Claws Shadow Sneak Banette-Mega, -ate Rayquaza with Extreme Speed, or even Steelworker Bullet Punch Kartana.
Sturdy Shedinja with Shadow Sneak does about 33%, and it can be a wear down tactic where you attack as the foe uses Entrain, switch out and send back in so that Sturdy stops you from being KOed until they remove the ability. Some use Foresight to bypass Normalize for the ally that switches in to always hit.
As someone whom uses Gengar with Entrainment there are not as many checks as there could be, but just enough for me to have to prepare.
Also, I have been Prankster Encored, Taunted, etc. and that is something that can be a situational check. They cannot just switch into Gengar bc they can lose their ability if they are switched into Entrain, but it can be a slow pivot in.
Also, having Haze or Topsy-Turvey, regardless of ability, on a Audino-Mega, or Pokémon not weak to its moves is a way to Stall and prevent set up without Unaware.