Put -dxlevel 91 in your launch options, unsubscribe from all workshop addons (excluding PERP), completely empty your addons, download, downloads and downloadlist folder in your Garry's Mod directory, type 'menu_cleanupgmas' in the console to get rid of unused .gma files (addons) as soon as you open up Garry's Mod again. Another thing you can do is update your drivers, I had the same problem where Windows 10 on my laptop fucked up every single driver of mine and couldn't join PERP with the same error message that you provided.