I don’t know what the manufacturer meta tag list is but from reading Manufacturer & Company tags are back, I gather that it’s a way of removing tags that aren’t particularly useful by replacing the tags with more meaningful one such as `intel-graphics instead of intel.
Given that there are only 3 questions relating to GNU/Hurd (which could arguably be referred to as a GNU OS), I don't think this tag is particularly useful for these few questions.
GNU programs are (mostly) compatible with the POSIX specification which standardises Unix-like operating systems. In many cases, the GNU tools extend the features specified by POSIX, e.g., GNU sed has a very useful -i, --in-place option that is not specified by POSIX and is not present in BSD implementations of sed.
Also, the program flags specified by POSIX are only one character (e.g., -q for a quiet) mode while the GNU version will usually accept long (more readable, multi-character) options in addition to the short options (e.g., --quiet and -q).
The main use case that I can see for the gnu that it signifies that a question and answer can refer to the extended features provided by GNU. E.g., tagging questions with both gnu and awk will allow answerers to know that the questioner is asking about the GNU version (on most GNU/Linux systems, the awk program is a symbolic link to /usr/bin/gawk) rather than the BSD implementation.
It’s probably easier to keep gnu rather than creating new tags for each of the many GNU programs (gnu-sed, gnu-awk, gnu-grep, gnu-find, gnu-tar, gnu-date, gnu-cal, etc.).