Busted Craft would be quite correct. Any fiction involving the real musicians behind the animated characters is properly Celebrity fiction, hence the Gorillaz category there.
I will assume that no one has requested a cartoon-based category for Gorillaz to date, nor has anyone decided to write real person fiction involving the musicians behind Dethklok. Should anyone want to write fictions for those as yet nonexistent categories can request a category be made. We have a thread and procedure for that purpose here: http://www2.adult-fanfiction.org/forum/index.php/forum/733-archive-category-requests/