All you have to do is set HDR to full. Go to your gmod video settings then click advanced and under High Dynamic Range set it to full and then click "ok". It will lag for a bit don't freak out but when it's done reload the map if you've already loaded it if you didn't already load it you should be able to go into it with no problem. If it's still fullbright go into the console and type sv_cheats 1 then type mat_fullbright 0 if that doesn't work then it's some CIA 5G problem with the map that I can't fix :(.