To be fair, the Gatomon in Adventure is a singular character of a species, which means that a feminine terminology would only be fitting of her and any other female versions of the species. Gato on its own can just mean a generic cat along with it being a masculine version, which means it covers all members of the species more broadly then the feminine only version would and wouldn't fall into Mr. Mime issues in the future.
I'll also be fair to Omnimon since it makes complete sense in the context of not only the movie, but the character since it's a fusion of two Digimon generically, and a fusion of the hopes of those watching the events in the movie. In fact, I'd go as far as to say it's better then Omegamon in that context, it's just that the developers eventually gave the Japanese name more context with Alphamon years later that made it make more sense while Omnimon ended up looking strange in retrospect (which is probably why N0 and Tri have since reverted to the Japanese name).