I made the default GS theme responsive with the Skeleton css grid and it works well.
You can see the theme in use unaltered here: http://timbowgs.bplaced.net/
It's a normal theme: Unzip, Upload, Activate
If you have images on your pages you will need to specify their dims in % to make them scaleable and fluid. You can do it through the GS Backend in the Page Editor, Image Properties, delete the image dimensions in pixels then go to the Advanced tab, in the Style box write a declaration like max-width: 50%; (for example).