Should also work in Textmate, I've copied the CSS package and edited it to give syntax highlighting for .gss files here:
You should create a new folder under Packages (you can get there by going to the sublime text 2 menu -> preferences -> Browse Packages....) called "GSS". Restart sublime text and away you go. So far there is highlighting for:
uppercase variables (hopefully you're only using uppercase for variables)
functions (both CSS function and @mixin functions)
@defmixin function variables
Any feedback about improvements would be great!