Exactly, this is a project built for Gzdoom that makes use of everything Gzdoom offers, When I make projects on Doom, I want to take them as far as possible away from standard Doom, and Gzdoom gives me the best freedom of choice, since it's at this point a standalone engine for videogames, look at projects like Selaco or Ashes 2063 for example.
When I develop GS3, I don't think i'm making a mod for doom, I'm in the mindset that I'm making my own game which happens to share parts of the doom universe, but in the end, it's a vastly different thing, and it's deeply rooted in what this new version of the engine can offer in contrast to the original.
It's alright, there's plenty of old school projects around for people to play, Golden Souls just doesn't happen to be one of them, and it's alright, because it's a beautiful thing that we can have so many mods in this community with different scopes in mind!
If someone goes and makes something akin to Golden Souls in style but with vanilla doom in mind, props to them, I may be the first one to try it, but that ain't my thing to work on.