I think that with this particular set (Sub + DD) you would probably want to invest a tiny bit into HP to gain an additional point of leftovers recovery, and due to that, I would change EQ to Bounce. Bounce will provide you with a secondary, decent STAB and a move that will give you even more leftovers recovery, helping your Gyara to maintain healthy thorought the entire fight.
I do like this idea, although I think I would want to add 20 speed EVs and take the defense EVs away- Skarm is in a vulnerable speed tier when it comes to walls and getting Taunted before you can Roost might be a big deal.
Yes, Gengar does get Nasty Plot in MMO- however, remember, it has Cursed Body and not Levitate here. When it comes to setup set, it usually carries Sub/ Plot/ Focus Blast/ Shadow Ball and carries leftovers. With Substitute Gengar can fish for Cursed Body proc (which is incredibly ghaey if I may add) and get further setup opportunities. Other than that, your specs set looks standard and very good.
I don't know if you want another setup sweeper in your team- you already have quite a lot of them. I think CB or even Scarf infernape would make more sense here as you could use speed control in your team and some momentum gainer through u-turn.
Pretty standard, decent Scizor set and spread, although you would surely want to run 252 HP on it (odd HP) and invest a tiny bit in speed as bulky SD Scizors are not uncommon and being able to win a speed tie with an uninvested SD Sciz can win you the game. You would probably also want to run either Careful or Adamant Sciz instead of Impish.
Garchomp unfortunately doesn't get Rough skin in MMO. The most common rock setter Chomp set is Naive/ Hasty with 252 satk/ 252 speed/ 4 atk with EQ, Rocks, Draco Meteor/ Fire blast or Flamethrower with Life Orb.
Overall it looks decent and I think that this team could work if we would just change it a tiny bit and probably modify later on, based on your experiences with it.
