The question can be answered in any of these ways:
A gon is an animal which lives under the sea.
The gon is an animal which lives under the sea.
"Gon" means an animal which lives under the sea.
"Gon" is an animal which lives under the sea.
The last is unlikely in conversation unless explaining a foreign word (and even then, the version with "means" is more likely).
If someone didn't know what a dog was, it would be bizarre for me to say to them, "Dog is a four-legged animal with a tail". Instead, I'd say "A dog is a four-legged animal with a tail" or (less likely) "dog means a four-legged animal with a tail" (though this would risk wrongly implying that the word "dog" applied to all animals that met that description - including cats!).
But if they were asking what "chien" meant, I could say "chien is a dog" or "chien means 'dog'" or "chien is French for 'dog'".
So, even though your question was "What's the meaning of 'Gon'?" rather than "What's a Gon?", I still think that the most natural way to answer the question is "A gon is ...".
"The gon is ..." is equally acceptable but sounds more formal and is more likely to be found in a textbook.