This Gourgeist is a ghost/grass-type Pokémon owned by Jessie from Team Rocket and the first Pokémon she obtained in the Kalos region.
Gourgeist, as a Pumpkaboo appeared in The Bamboozling Forest! while Team Rocket was lost where she appeared out of the ground after Jessie tripped over her horns. Jessie immediately threw a Poké Ball at her and caught her easily in one go.
Pumpkaboo was used to help Jessie compete in a Pokémon Showcase located in Courmarine City. But lost was she got the least amount of votes from the audience.
Pumpkaboo was later used to compete in the Pokémon Sky Relay tournament. But despite managing to win the first lap, she was disqualified, along with her team when Meowth's Pelipper machine was exposed.
Sometime later during the Gourgeist Festival, Jessie traded her Pumpkaboo in exchange for Count Pumpka's Mawile in hopes for Mega-Evolving her and for Prince Pumpkaboo's happiness. However, when she saw the results of her Pumpkaboo evolving into Gourgeist during her trade and Prince Pumpkaboo lost full interest in her, she changed her mind and traded her back.
Not much is known about Gourgeist, but she has a habit of flashing the lights in her stomach. It also shows that she has no problem listening to Jessie's commands.