"you have to go to "Settings" then System then Screen i think because i´m from Germany and stuff. (Screen is already selected if you go to Setting then system.)
Next scroll to teh setting called " advanced display settings" then click "Display Adapter Properties" then click monitor and change the "hertz" instead of 59 to 60 .
And thats it you can enjoy Garry´s Mod again

the reason is the latest windows 10 Update i think.
I hope i can help you with that
if it worked could you let me know then?"
I had the problem with a lot of source engine games, this worked for me. Ik it says garry's mod but iirc CSGO uses the same engine