I had this error.
Might not work!!
Start Hammer with CSS.
Remove your GMod settings.
Add new settings, name it GMod (Duh)
When you're selecting fgd's go back and select orangebox then bin then select the fgd's you want.
Restart Source SDK and Hammer.
Select Orange box then GMod then click Hammer.
If it doesn't work then it's b/c of the update.