GT -is- Fanfiction, because it's not created by the original author, and as such, you can't judge whether something is true or not in DBZ, by whether it is in GT. GT is not made with the creator of DBZ's artistic vision, it is made by a plethora of random people--hence it is fanfiction. You're just arguing that because a legitimate company produced it, it's fanfiction.
If -anybody but the original author- adds onto a story, with permission or not, it is fanfiction. It doesn't matter if that fanfiction was then turned into an anime. It's still fanfiction, because the story it's based on had a different creator.
You can argue whether it's fanfiction or not, but it boils down to exactly what I said--nothing in GT can prove anything in DBZ wrong or right, and it doesn't have Toriyama's artistic vision.
As for the copyrighting..
This section is a bit vague, but this is where the right to write your own story regarding existing shows/books/movies is protected.
"Copyright Management Information - New Section 1202
The DMCA prohibits tampering with copyright management information (CMI). Specifically, the DMCA creates liability for any person who intentionally provides or distributes false CMI. 20 Also, the DMCA prohibits intentional removal or alteration of CMI, and knowing distribution of illegally modified CMI is similarly proscribed. 21 To be covered by the DMCA, CMI must be conveyed in connection with a copyrighted work and CMI may constitute any of the following:
(1) information that identifies the copyrighted work, including the title of a work, the author, and the copyright owner;
(2) information that identifies a performer whose performance is fixed in a work, with certain exceptions;
(3) in case of an audiovisual work, information that identifies the writer, performer, or director, with certain exceptions;
(4) terms and conditions for use of the work;
(5) identifying numbers or symbols that accompany the above information or links to such information, for example, embedded pointers and hypertext links; or
(6) other information as the Register of Copyrights may prescribe by regulation, with an exception to protect the privacy of users. 22 "
As long as you give proper credit to where the ideas came from, (i.e., author and the company who produces the animation) you can't be prosecuted for copyright violation.
Edit: I also agree that under this act alone, Fanfiction is still technically illegal, but the information written here, added with the digital privacy act, ensure Fanfiction's legality, and that no one who writes fanfiction will ever be prosecuted.