Gill-Man is the Creature from the Black Lagoon.
The New Scooby Doo Movies
A differently designed purple version of Gill-Man appears in "The Externimator" as one of Lorne Chumley's costumes.
The New Scooby-Doo Mysteries
Gill-Man with other party-goers
Gill-man was one of the monsters in Dracula's party. (A Halloween Hassle at Dracula's Castle)
Also, in the intro Gill-Man can be seen dancing with Frankenstein, Igor, Dracula, Wolfman, and Medusa.
Scooby-Doo And The Ghoul School
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Gill-man and his daughter
Gill-Man was with his daughter (so that she could become a new student at Miss Grimwood's Finishing School for Ghouls), but the sight of her (and the other new students) scarred off Scooby-Doo and Shaggy Rogers, who had had enough.