A bit of grave digging but :
The translation for gillman is 1) too long, 2) refer to a specific creature when we are talking about a species, 3) is totally out of context : what black lagoon. Cinematographic reference use is correct for pop culture reference, not for obscure references no one will have.
If we were talking about Men in Black or MiB like in XCE, every french speaker know what are Men in Black, but no one have an idea of what the 'Créature du Lagon Noir' is, especialy when it refer to some unknown location never mentioned in the game.