Have you tried restarting your game? I would also recommend uninstalling the old Prometheus content pack if you have it and clearing your Steam download cache as said in the Discord post by Verax.
If it doesn't work try removing everything in your Gmod addon folders, specifically in the "downloads" and "addons" folder (right click on gmod on steam, properties, installed files, browse dont actually delete the folders just the contents of those two aforementioned obviously) unsubscribing from all steam workshop addons. If it still happens you can try to factory reset you gmod, in the main gmod folder double click "FactoryReset-GMod.bat". If all else fails you can do a full reinstall, that is uninstalling the game from steam, deleting the gmod folder from your steam files manually and also disabling steam cloud in the steam properties for the game as that could try to carry over some old files. Keep in mind that these last two steps will remove any locally saved content.