Or you could just contact the author and ask him to upload it to nexusmods or wherever right? Why all the mental gymnastics around this?
The Nexus currently has zero working mods for gamemaster mode. No one is adding them for some reason.
That is pretty messed up by I'm guessing that is because the game came with Steam Workshop support and what's more, that support is "baked in" to the game itself. Meaning you can upload content right to Steam. Nexus is taking a beating because of that.
The best thing you can do is ID content you see on Steam Workshop and ask the authors to move it over. It is like a 2 min process to zip the file and upload it so I'm guessing they wouldn't mind.
As for lack of content in either place... people are still jamming through the single player campaign, I think in a few weeks once that is over you will see a massive spike in GM centered content. We can hope so at least if we want a Neverwinter Nights experience and not a garbage Sword Coast Legends experience. Time will tell.