What effect did Pokemon have on the metagame?
Explain how the Pokemon effected the metagame as whole, and how the metagame adapted around it. A brief description of which Pokemon it countered and which Pokemon it did well against would be good here as well. Be sure to consider their impact in previous iterations of SS STABmons as well if they weren't removed.
Gengar has had a weird history in Generation 8 STABmons. While it existed from the beginning of the generation, until about a month before DLC 1 dropped it was overshadowed as an offensive Ghost-type by Dragapult. This isn't to say it was bad; but Dragapult was the dominant Ghost and one of the "big four" Pokemon used in that period of the metagame, while Gengar sat lower down in viability. Once Dragapult was banned and DLC 1 dropped it very quickly became apparent that Gengar was too strong for the metagame as it stood. With tools such as Shell Side Arm and Moongeist Beam its Nasty Plot, Specs, and Choice Scarf sets were too varied and too strong for the metagame to reliably answer.
With DLC 2 Gengar was freed joined several other ghost-types in immediately spooking the metagame. Tyranitar or a blob was seen as necessary to deal with Gengar, Spectrier, and Blacephalon. Soon, Astral Barrage was removed from the metagame by a council vote split between removing the move and removing Gengar and Blacephalon (Spectrier having already been banned). Blacephalon was eventually banned, and another competitor of Gengar's, Aegislash, left around the same time.
Gengar now has nearly no competition left in the tier. It is capable of blasting through most defensive Pokemon in the tier, although in the current, Speed-heavy metagame it finds itself a little less than top tier. Nevertheless over the course of the generation it has stood with the other top tier Ghosts, outlasting them all and pushing the metagame into running constant counterplay (
) for them.
In what main roles was Pokemon used?
Gengar utilizes its excellent typing and high Speed to perform various offensive role, including being a setup sweeper, abusing choice items, or stallbreaking.
Generation 8 gifted Gengar with Nasty Plot, which boosted its sweeping potential greatly. This combined with its excellent Ghost/Poison/FIghting coverage options and, as always, its excellent statline allowed it to work as one of the top special sweepers for much of the generation. Besides this, its Ghost/Poison typing allowed Gengar to set up on certain Pokemon very effectively. The existence of Pokemon utilizing mono-Normal movesets, or attempting to utilize Body Press as their main attack (or either of those plus the 4x resisted U-Turn) gave Gengar many opportunities to either Nasty Plot or set up a Substitute.
What caused it to have a significant impact?
Gengar's power, offensive flexibility, and the difficulty of revenge killing it all contributed to the impact Gengar had on the metagame. Because of the unreliability of Gengar's few checks and counters many teams have been pigeonholed into running Tyranitar for most of the generation. The flip side of this is that Tyranitar has been near omnipresent for the entire generation (not just because of Gengar), making Gengar a risky choice at times due to how common its best answer is.
Teams have to be prepared to tank Specs sets, not be cleaned by Scarf sets, avoid and pressure Nasty Plot sets, and be able to break Substitutes with their walls. Gengar being able to do any of these is terrifying, and a misprediction can lead to a lost mon, if not simply a game over.
How do/did you deal with this Pokemon in STAB?
One of the main things that make Gengar so fantastic in STABmons is the lack of Ghost resists. Between all of its sets it really has no truly reliable counters that will eat a hit. However there are ways to play around it due to immunities to all of its common moves and Gengar's extreme frailty putting it in poor positions if it does not predict perfectly.
Tyranitar is, and has been the best Gengar check and counter for the entire generation. Chople Berry Tyranitar can switch into Gengar once per match no matter what, and the types of teams Tyranitar typically fits on should not need more than that, especially since Gengar is incredibly frail and should not be given many opportunities to come in. Non-Focus Blast Gengar does not threaten Tyranitar significantly. Blissey also serves as a counter to most of Gengar's sets, although it must be wary of Trick or Taunt, and has to carry Shadow Ball. Mandibuzz is perhaps the most reliable Gengar counter, but is generally passive in a fast metagame and overall a poor option.
Despite Gengar's excellent speed it is still vulnerable to faster threats or priority. Tornadus-Therian, Tyranitar, Zeraora, Weavile, or various Choice Scarfers can all pressure Gengar due to its frailty, including its weakness to Knock Off and Sucker Punch.