Gourgeist can only be obtained as a Pumpkaboo.
But, when encountering Pumpkaboo, it will be completely random as to what size it will be when you catch it. So just keep running around in the tall grass until you find a Super Size Pumpkaboo, then evolve it.
However, you can try lead with a Compoundeyes Butterfree since according to this all Super Size Pumpkaboos hold a Miracle Seed. So you can try do that, however the creator of the forum said there was no difference. You may have different luck though.
When you find the Super Size Pumpkaboo, evolve it into Gourgeist. It won't change form when evolving, so don't worry. :D
Hope I helped. :)
Source: Experience and Above Link