I am sure the game could easily still be put on GOG, but it's still a buggy mess and Otherside, I theorize, is putting more of its resources into System Shock 3 currently. I imagine more so GOG is waiting for the game to be more stable and all. Now I know there are unfinished games on GOG that are still in development, and some games do need fan patches but you need to understand that GOG is just one team and it takes a developer to remaster or update a game sometimes, they can't do every single game. an example I have is Rainbow Six 1(the original), it needs a fan patch to work but it isn't a massively demanded game to be remastered even though I would gladly love that.
Just remember, there are publishers out there that are so fickle, EA cough cough, with their "property" they will keep a game on cold ice for years, decades even before they let anyone remaster it or fix it up. And yes, atm EA is supporting the C&C remasters but think of how long it was since the First Decade pack and all the shit they did to C&C over the years. Hel, THEY are the ones they wouldn't let Arkane or Otherside do a sequel to the Ultima Underworld series in the first place. We will never 100% know for sure who and what has said this and that or what deals were said. I wouldn't put all the blame on GOG for this one, cause Underworld Ascendant has gotten lots of negativity for a reason outside of backers.
We'll see what 2020 brings but I wouldn't get down on thinking it will never come to GOG, honestly Otherside needs to pull a Sean Murray Hello Games move and focus on the true criticism from people who actually play and bring the title in what it truly is suppose to be. I wouldn't doubt GOG is worried about bringing the title hear but again people will say they have for worst titles.
Here's to it joining the library