I always had this doubt about whether Dragon Ball GT was an official, canonical sequel of Dragon Ball Z or not. I know that Akira Toriyama helped in the development, but did he write any of it?
Dragonball GT was not made by fans. It was made by Toei Animation with involvement from Akira Toriyama. According to this wiki, his involvement is outlined as:
Akira Toriyama is credited as author in the ending credits of Dragon Ball GT; he oversaw the series' production, this was the same process that was used during the production of the anime series Dragon Ball and Dragon Ball Z. He drew a rough design for the GT logo, he designed the GT appearance of the series main cast, and he designed the appearances of Giru and the GT spaceship used in the Black Star Dragon Ball Saga. He also drew at least three color pictures of Goku, Pan, and Trunks adventuring on various planets (Monmaasu, Rudeeze, and an area in Hell).
Toriyama seems to have positive feelings towards his works' continuation, as he drew his own version of Super Saiyan 4 Goku (which was originally designed by Katsuyoshi Nakatsuru) exclusively for the Dragon Box GT. Characters and events from GT have also been included in more recent Dragon Ball video games.
Dragon Ball GT was produced by Toei Animation, one of the anime companies used to produce Dragon Ball Z.
Akira Toriyama did not write any of the episodes though, that's true. They're all fillers.
Dragon Ball GT was produced by the same companies as Dragon Ball Z, so technically it can be considered a sequel.
However, there is one 'but': this is more of an OVA than an adaption of the manga; in fact, Dragon Ball GT was never a manga. So in anime terms: yes, Dragon Ball GT is the official sequel, but not in manga terms.
Dragon Ball GT was done by Toei Animation and Akira Toriyama was involved with the drawings of the main characters (Goku, Trunks, Pan, Vegeta, Bulma, Gohan, Chi Chi, etc), the planets (Monmaasu, Rudeeze, Planet M2), and even the spaceship. Even though the GT series was non canon, and its not in the manga. I still think the GT series did okay (even though it has some flaws here and there), and of course its a sequel to Dragon Ball Z. Now we are on Dragon Ball Super, and I just have to wonder if the DBS series is going to go pass the end of DBZ (age 784), and go straight to the GT timeline (age 785-790).
Akira toriyama never said dbgt was non cannon remember besides with all the universes and timelines dbgt could be considered as a path the z fighters could end up on by making certain choices therefore creating an alternate timeline and a new universe the same with db super it is a result of another timeline created by the choices of the z fighters. The choices they make effect the timeline like when trunks came back in time and made an alternate timeline his still exists as does theirs so there are 3 alternate futures trunk's future,the dbgt future,as well as the db super furure. Besides will all these movies and television series and manga and comics idk who can tell what the heck is cannon anyway just say it's an alternate future that exists in an alternate timeline