Should we delete this tag, or keep it?
I have no question that tags about specific variants (maybe even builds) are ok. I just wonder whether this specific one isn't too redundant.
This tag is mostly redundant and of little use. The vast majority of questions will apply to gnu-emacs.
Of course, there may be questions which pertain only to specific variants, so we could take this tag to mean "specifically not variants". But none (or, arguably, one) of the questions currently tagged as gnu-emacs fall into this category.
This indicates the tag is likely to be misused much more than it will used accurately.
Furthermore, if we follow askubuntu's example, they do not have an ubuntu, while they do have tags for the variants.
This tag is meaningful, but it should only be applied to questions that are specifically about GNU Emacs.
Given that there is hardly any viable alternative to GNU Emacs, we could decide to do without this tag, due to the high risk of misuse. Most questions implicitly apply to a reasonably recent version of GNU Emacs anyway.