This episode marks the first time Ash has officially caught a Ghost-type Pokémon. Also, with Gengar's capture, Ash has now officially owned at least one Pokémon of all 18 types, including Pokémon whose typing has been changed since their introduction.
Ash catches the Gengar that had been haunting the Cerise Laboratory.
Gengar being abandoned by its Trainer who told it to wait for him, while they had no intention of ever returning, is similar to how Ash's Charmander was abandoned by Damian. Additionally, the shirt Ash wears at the end of the episode has a Charmander on it.