So after upgrading from Windows 8 to Windows 8.1 a few days ago there were some teething problems.
As the Post heading suggests Garry's Mod and other Half-Life 2 engined games were launching to blank screens.
First thing I did was check the integrity of the local game content. This didn't work.
I cleared the game config files to make sure there were no rogue gfx display settings. This didn't work.
Then I re-installed the Nvidia drivers and it worked!