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 Duwould 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.