Hello since the new update to gmod like always issues arrive with the updates. Well I didn't have any issues with updating but it seems that the new update broke some graphic settings as I was having my screen flashing white when I would turn and messing with the in game settings if it wasn't the flashing it would be when I turn around some areas would change their lighting in annoying way. Well messing with the settings if you are having the issues go to options in game and turn shadow detail to low. I tried to use medium instead of high but I have the same issue with the screen flashing . I hope this helps out and for reference I am running a I-7 5930k with 8gb ram, msi r9 390, windows 7.

If you don't care to read all that then Shadows detail in setting to low.