Open regedit (windows registry editor)
Navigate to HKEY_CURRENT_USER > Software > Valve > Source
Delete either the garrysmod folder or its’ settings subfolder, it doesn’t matter which.
Launch GMod, you’ll find all of your graphics settings have reset. Set them up again, launch game, post results.
If this doesn’t work, all I can suggest is that you uninstall your drivers, get Driver Sweeper from guru3D, launch windows in safe mode, remove remnants in DS, restart and install new drivers.
Also, I’ve never known source to not play well runnings HDR and AA at the same time. I do it on a regular basis.