I'm not the one to ask, but my efforts to customize, and alter the game were fruitless. I was unable to alter the ini files for some games, event though in the steam launcher you could go to manage local files. I could not find that option in GFN - it could be there and I just didn't put enough effort into ti.
As for mods, there's no way to launch the installers or manually move the files to the GFN managed games