The [tag:gnu] wiki excerpt defines the tag as referring to the GNU OS.  The wiki says the tag can be used for any software developed by the GNU Project.

As far as I know, there is no working GNU OS, at least not in mainstream use.  The OS-related GNU software is incorporated into many common distros (often referred to as GNU/Linux or just Linux).  The GNU Project has a bunch of other software, many of which have dedicated tags.  Skimming the 146 questions tagged [tag:gnu], I didn't spot one that was actually about the GNU OS.  It appears to be mainly decoration.

Recommendation: add [tag:gnu] to the manufacturer meta tag list.  If there are a few questions about GNU software that don't have a dedicated tag, we can create them.