It was pointed out in comments at Rename tag "Naruto" to "Naruto-Shippuuden" that we have separate tags for dragonball-z and dragonball-gt, and we'll probably end up with a dragonball eventually. However, in the case of Naruto, one tag naruto is encompassing the whole series. (Also, Dragon Ball has a space in it, which was not included in the tags for whatever reason.)
This is inconsistent, since Dragon Ball was a single manga from which both the Dragon Ball and Dragon Ball Z anime were made (at least in Japan; in English-speaking countries). It would seem that we should use dragon-ball for all of these questions, with dragon-ball-z as a synonym. GT is a bit strange, but at least based on my logic for the linked question, being part of the same franchise and story, it should also be included in dragon-ball.
However, even though this is consistent, I get the feeling that new users will have trouble with it. In principle making the synonym should fix things. However, I get the feeling that people will have trouble with it.
Should we keep it separated (thereby using a different convention from Naruto and some other series) or should we move all of these to dragon-ball and make dragon-ball-z and dragon-ball-gt synonyms? Either way, we should probably switch "dragonball" to "dragon ball" in all of the tags.