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I don't know how GF would work out making a Gourgeist Mega because of its 4 formes but I'll base this off the Supersize stat distribution.
Mega Gourgeist
Grass/Ghost
Flash Fire
HP: +0 (85)
Att: +35 (135)
Def: +20 (142)
SpA: +0 (58)
SpD: +30 (105)
Spd: +15 (69)
Gets a little bit of speed to better handle shit like Azumarill and Tyranitar. Now capable of being a great mixed wall instead of getting 2HKOd by Specs Pump from Keldeo.
Ideally I think it deserves Fire Fang or Flare Blitz, but I don't think there's even any way to get it through breeding because it's in the amorphous egg group.