Actually...
Dragonball Z continued to the end of the Dragonball Manga, which concluded with the defeat of
Majin Buu and
Son Goku's introduction to
Uubu, Majin Buu's reincarnation.
(if you must know,
Toriyama wanted to end it with the
Freeza saga, but was so popular he continued the manga to the end of the Majin Buu saga)
Dragonball GT is
Toei Animation's extension of the Dragonball series (as are all the movies), and it's believed to stand for
Goten and
Trunks.