There’s few ways to root a computer through gmod. Some depend on your OS, Some depend on addons, some depend on gmod version.
Either way, it sounds like someone’s being an asshole. Most likely the actions are executed from within garrysmod. So this means that it’s either lua based(lua -> X -> windows services) or module based(something like gm_rawio as said, You can scan existance of the support from the client thus allowing the admin to know weither or not your computer can be rooted through it).
The lua based ones are mainly limited to windows XP, However with all the nowdays stuff added to gmod the lua based ones can be global aswell and in somecases even using the user level account won’t save you. So don’t count on the fact that you’re using win7 or vista.
If the commands come from garrysmod then the antiviruses and other programs are prety much useless for anything that works on that level, as those organizations are not aware of the potential danger. You just got to remove the garrysmod folder and hope that the guy rootting you didn’t create anything low level or hybrid crap to maintain the control.