Sucy/10
And that's not a bad thing.
In a fight, mine's a goddamn monster.
Despite being small, she tanks hits like nobody's business.
The size gimmick is actually pretty neat, and well-implemented in the game itself.
Grass/Ghost is a pretty great typing too, and Trick-or-Treat into shadow sneak is one hell of a combo, though pretty easily telegraphed.
Trick-or-Treat itself is also a really good move for more than just giving the target weaknesses, too.
Heck, Trick-or-treating some party members in doubles, such as Weavile or Bisharp completely mitigates their crippling fighting weakness, so Gourgeist also makes a pretty nice support mon in doubles as well as a tank, or even a sweeper if need be.
Overall: 9/10, favorite gen6 mon by far.