These are the l10n style guides which are given to translators by Sun when they are translating Sun software. These are actually Sun's Solaris style guides but they should be generic enough to apply to any project (including GNOME).
The style guides were originally written in Frame Maker but have been exported to .mif format so some enterprising soul should be able to convert them to docbook.
Currently the source is stored on the Gnome CVS server and can be found here. It is in Frame Maker and .mif format so there should be tools out there which can edit/convert these.
If you wish to check it out use gnome-i18n/l10n-style-guides as the module name (i.e. 'cvs co gnome-i18n/l10n-style-guides').
Contact michael.twomey@sun.com if you have any questions on the style guides.