It is based on the Jack-o-Lantern and poltergiest.
It's name is a combination of "gourd" (an alternative to pumpkins when carving Jack-o-Lanterns) and "poltergiest."
It has a Jack-o-Lantern face on its body.
Phantump and Gourgeist are the only Pokémon that can be found with different sizes in the games.
Gourgeist's HP stat increases by ten as it's size increases (55 small, 65 average, 75 medium and 85 large). It's attack also increases by five as it's size increases (85 small, 90 average, 95 medium and 100 large). However, it's speed decreases by fifteen as it's size increases (99 small, 84 average, 69 medium and 54 large).