I am not sure if I am following you 100%, but you can add another buffer and processing step into your pipeline to create that extra pass to create a Gaussian blur. You can use the depth buffer to filter out objects from the background (check the wiki).
However, another approach would be to not use Gauss blurring and instead use a cheap single pass blur filter, if that makes it easier for you. For that just grab the 4 surrounding pixels and average them in the shader. Here is a more complicated example:http://www.pasteall.org/10779