I have mirrored GSB (GNOME SlackBuilds) repositories into our repo server. It's not listed in the main page, but still it can be accessed via HTTP and RSYNC:
So far, i only mirrored GSB 2.28 and GSB 2.30 for Slackware 13.1 and Slackware64 13.1. GSB 2.28 is considered final, but 2.30 is at RC1 status, so it's not yet final, but the developer stated that they will make a final status in short time.
The repo doesn't consists of the sources, so if you need sources, then you might need to add other repositories.
Tutorials about how to download and install GSB can be found on the Download and Install page.
WARNING: Don't install the GSB above on Slackware other than 13.1 or newer since it will not work. Also read the documentations first before trying to upgrade or install GSB as there are some packages needs to be replaced in order for GSB to work