You'll need to get it from https://build.rockbox.org/ . We never got the installation smooth enough to consider it stable enough to "release", so it's not on the release download page.
Actually, you can partially use Rockbox Utility for installation:
- manually install the bootloader (that's the main step missing in Rockbox Utility)
- Go to the configuratino dialog of Rockbox Utility and enable the "Show disabled targets" checkbox.
- Now you can select the Gigabeat S and install.