Agatha's Gastly and Haunter intercepted Professor Oak in the halls before the finals of the Indigo League began. Gastly and Haunter attacked his Kangaskhan and Chansey, who retaliated back and wounded Gastly and Gengar. The two were also sent during the finals, but Oak managed to defeat them and become the Champion of the League.
Agatha was waiting at the power plant with her Ghost-type Pokémon, including Gastly. Gastly and Gengar emitted fog to drain Blue from his energy. However, Blue broke free and had Scyther slash Gastly and Gengar at their cores, defeating them.
After Red battled Bruno, the latter was joined by Agatha and Lorelei, who asked of Red to join the Elite Four. Since Red refused, the trio had their Pokémon, including Gastly, create a huge blast that hurt Red and his Pokémon.
Agatha had Gastly attempt to take Miles away, who knew some information about Red's whereabouts. As Blaine had Growlithe launch Fire Blast, Gastly moved to a tree, which became burnt by Growlithe's attack. Gastly took control of Miles to attack the Gym Leaders, but was defeated by Blue's Charizard.
While speaking to Blue, Agatha explained they managed to capture Red by combining the three Pokémon's types: Lorelei's Jynx, Bruno's Hitmonlee and her Gastly. She also admitted she had Bruno wear a special band that allowed her to control him and order Hitmonlee to attack Bill at his house. After explaining things to Blue, Agatha vanished with her Gastly.
None of Gastly's moves are known.