Actually, Jolly Garchomp is generally favored over Adamant in Doubles unless if it was Choice Scarfed, mostly to Spam Earthquake more effectively. Use Poison Jab instead of Iron Head, it hits Fairy Types just as well and hits Grass Types too. Use Dragon Claw or Fire Fang instead of Outrage, even if you have a Persim Berry, Outrage still sucks in doubles and may hit your Teammate. Use Groundium Z on Garchomp to take out bulkier threats it wouldn't otherwise. :P
Tapu Koko is frail and generally won't be able to use Roost as it needs survive most attacks without losing half of it's HP. Grass Knot should be more useful to Koko than Roost.
Gengar is very situational- not every Pokémon can be poisoned, Toxic can miss, and you could get Taunted. They can also switch out after being Poisoned and cure it/avoid being damaged by Poison/Ghost moves. This strategy is essentially stopped by Dark, Steel, And Poison Types, and Destiny Bond is countered by Will-O-Wisp, Priority Moves, and Taunt :P
Mega Metagross can benefit from better moves than Protect, Hammer Arm, and Zen Headbutt, like Bullet Punch, Ice Punch, Thunder Punch, or Earthquake, for example. (The Priority is worth having 2 Steel STAB Moves).
Then again, I could be wrong... :P
