Guten Tag as a closing salutation is by far not as widely used as Auf Wiedersehen, which should be fine for all purposes where you would also use Sie instead of Du.
For a generic salutation that is more informal, you could use "Tschüss!", which I would translate as "See you!" or "Bye!". In southern Germany (especially Baden-Württemberg), there's also "Ade!" (emphasis on the e), and younger people often use "Tschau!".
Tschüss, Ade and Tschau have the same roots, originally even with a religious connotation (french adieu, "be with god" → atschüs → tschüss), but that is long gone.
