Gourgeist is a ghostly Pokémon that has four sizes: Small, Average, Large, and Super Size. Its body is a dark brown pumpkin with a glowing, yellow-orange face carved into the front and a pair of nubby legs. The face consists of two triangular eyes and a grinning mouth; on top of the mouth is a small, triangular cutout for a nose. Poking out of the top of the pumpkin is its slender brown-orange head, which has bright yellow eyes and a small mouth but no visible nose. Gourgeist has long, light pink hair with arm-like tendrils framing its face and bangs that usually cover one eye. On top of its head is a crescent-shaped tuft that is brown on top; this tuft resembles a pumpkin's stem.
On the night of a new moon, Gourgeist wanders town streets while singing an eerie song. Those who hear its song are cursed. It is also known to sing joyfully while observing prey is has entangled in its arms. Gourgeist and its pre-evolution, Pumpkaboo, are the only known Pokémon that can learn Trick-or-Treat.
A Super Size Gourgeist has a different cry from its other three forms, pitched lower than the cry shared by the other forms.
Only Average Size and Super Size Gourgeist can have its Hidden Ability Insomnia. Average Size can be obtained as a Pumpkaboo with its Hidden Ability from the Friend Safari; Super Size can be obtained as a Pumpkaboo from event distributions.
Gourgeist appears to be based on both gourd and modern pumpkin jack-o'-lanterns. Its multiple sizes may be a reference to the tradition of trying to grow the largest pumpkin for Halloween. Its upper body may also be based on the candles within jack-o'-lanterns themselves, with the curl being its wick and its hair-like extensions its dripping wax. It could also be viewed as a mere extension of the shape and the curl on top being its stem. The lower body's different color might also reference the tradition of decorating gourds with paint.