Try going into your Steam Library, right click Garry's Mod and then click 'Set Launch Options', then type "-dxlevel 95" (without quotation marks) and try to join the server, this worked for me and I still have it in my launch options today.

Try going into your Steam Library, right click Garry's Mod and then click 'Set Launch Options', then type "-dxlevel 95" (without quotation marks) and try to join the server, this worked for me and I still have it in my launch options today.