The 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 gnu, I didn't spot one that was actually about the GNU OS. It appears to be mainly decoration.

Recommendation: add 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.