I think it's time to
drop Gengar from C+ to C. The meta is just too fast and too bulky to use Gengar for any reason. It's outclassed as a breaker by Blacephalon and as a cleaner by Ash-Greninja and Alakazam. Speaking of Ash and Mega Zam, they are ubiquitous and easily OHKO it. Scarf Lando is also extremely common. While Gengar can in theory run some interesting tech moves, it's really just not doing anything anymore. Gengar struggles to break the ever common Celepex cores in light of Zygarde's ban. I challenge anyone to build a team that wouldn't be better served by the aforementioned special attackers. In practice I think the best thing Gengar can possibly do in a match is perhaps trick a scarf onto a Pex. Maybe hitting something with wisp? (Blacephalon can do both of these things I might add!) I also don't think Gengar is as viable as its fellow C+ members, as each of them (barring regular Latios I guess) have a distinct niche that can't easily be replicated. The decreased viability of Tangrowth in light of Zygarde's ban is also detrimental to Gengar, as that was something that could be hit with a Sludge Wave (protean Greninja is the best offensive poison type in OU lol).
I would definitely disagree with some of your reasoning on Gengar. First of all, it doesn't lose to celesteela + pex if it's running tbolt and taunt. Toxapex can't do anything back and there's a chance of scald getting disabled which will usually result in a struggling pex. Additionally, it can run trick which completely ruins pex and celesteela. Though taunt isn't as good at stopping cele, you can run electruim z which will definitely kill due to a lack of reliable recovery.
252 SpA Gengar Gigavolt Havoc (175 BP) vs. 248 HP / 232+ SpD Celesteela: 278-328 (70 - 82.6%) -- guaranteed 2HKO after Leftovers recovery
One thing gengar does have over Blace is a higher speed stat. This means that if it runs choice scarf, It can outspeed scarf kartana and outspeed scarf blace. This makes it one of the best revenge killers for mons such as mega zam, kart, blace, and due to it's poison type it can revenge kill all the tapus. It also works against keldeo which blace cannot outspeed. There aren't really many good switches which can stop gengar since there are a decent amount of sets it can run. Chansey can't damage blace and most mons that rely on toxic can't really do anything. There are barely any ghost resists in ou, and those who do resist it are weak to fighting. Gengar does get focus blast, and assuming you don't miss it can be really scary switching into gengar. Some of the most viable mons in ou lose to gengar which is why having it drop is not needed.
252 SpA Gengar Sludge Wave vs. 248 HP / 0 SpD Tapu Fini: 248-294 (72.3 - 85.7%) -- guaranteed 2HKO after Leftovers recovery (not an OHKO but thats scarf and fini can't hit gengar back and blace gets walled by this)
252 SpA Gengar Shadow Ball vs. 4 HP / 0 SpD Kartana: 315-372 (121.1 - 143%) -- guaranteed OHKO (easily one of the best revenge killers)
252 SpA Gengar All-Out Pummeling (190 BP) vs. 252 HP / 0 SpD Heatran: 396-466 (102.5 - 120.7%) -- guaranteed OHKO
252 SpA Gengar Sludge Wave vs. 0 HP / 4 SpD Tapu Lele: 276-326 (98.2 - 116%) -- 87.5% chance to OHKO (if you have scarf can revenge kill scarf lele)
Sets
Gengar @ Fightinium Z / Electruim Z / scarf / specs
Ability: Cursed Body
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Shadow ball (Mandatory move)
- Focus Blast (should be used most of the time)
- Taunt / thunder bolt / sludge wave
- Destiny bond / Energy ball / Will o Wisp / Trick
Specs and electruim Z aren't that viable in ou but can still be used, moves at the bottom are mostly meant as surprise gimmicks but otherwise you would mostly use taunt / tbolt there if you don't already have it
Edit: forgot to mention that its much easier to revenge kill blace due to it being weak to aqua jet. This lets mons like azu and crawdaunt to kill it(water shrunken too and that breaks blace substitutes). Also ttar makes blace useless while gengar can z move it