Subject: I suggest following the sources.
Posted on: 2022-07-21 12:31:27 UTC
Fanfiction.net has a section for "Game of Thrones", but not for Song of Ice and Fire; that suggests GoT is open for fanfic. On the other hand, they don't have a section for "Interview with the Vampire"; it looks like Anne Rice requested the removal of stories featuring her characters, rather than fanfic of her books.
Per Wikipedia, GRRM considers fanfic to be copyright infringement. He's wrong, but that's the basis for his ban - so who owns the copyright to GoT? My guess would be HBO, who have not requested a take-down.
Given that there is no solid legal basis for banning fanfic, we're left with the creators' wishes. If they don't want people playing with their characters or worlds, that would cover adaptations. If they just talk about copyright, then only things they still own count.