GMod won't start

it’ll flash a window for a second
it does not generate a crashlog
I have reinstalled garrysmod and deleted my garrysmod folder
I think this might be an issue with my new screen
I had garrysmod run in a window with a resolution that is no longer supported

I have the exact same problem!!

is there any way i can delete the games configuration? It seams that steam now saves it even if you reinstall
found out how to fix it.

Try downloading the latest version of DirectX

Do -dxlevel 9 to see if it starts, if it does, your problem is directx based and then try -dxlevel 80 -windowed (not sure what is command to set window mod)

I just ran -autoconfig once

Are you the administrator of your computer? If this is vista, Run running as an administrator. Another thing you could try is running it in Winows XP compatibility mode. Let me know after you try these and I’ll tell you more options to fix this.