Afaik, GAMMA and most STALKER mods are on Moddb, not Nexus. It's one of the main reasons GAMMA uses its own installer to streamline the process. If it was on Nexus or the Steam workshop it would've just gone with said host's built-in mod collection approach instead.
They're already halfway there. They got their hooks in Nexus, which is the primary host for mods, and I'm sure they've infiltrated the competition.
Afaik, GAMMA and most STALKER mods are on Moddb, not Nexus. It's one of the main reasons GAMMA uses its own installer to streamline the process. If it was on Nexus or the Steam workshop it would've just gone with said host's built-in mod collection approach instead.