Why kill off Gorgon?
It seemed very forced to me. Not just poorly done but as if someone said write out a character because we have to be like that zombie show doing well or that throne game show thingy.
Or did he quit? Or worse, fired?
I don't know. They killed off Triton in the first episode. I don't know what the Inhumans comics are like these days, but back in the day Gorgon and Triton were always part of the team.
Ordinarily, I'd say that the show may figure out a way to bring back both: Gorgon with some DNA from the regenerate girl, Auran; and Triton fell in the ocean where he would be stronger - so he might heal and wash up to shore.
That being said, there are dim hopes for any more episodes beyond this year. The show has both terrible ratings, and bad reviews.