Well, maybe it's not so much greetings themselves, but their constituent words. It's Guten Morgen, Tag and Abend, because those are masculine nouns. And Gute Nacht, because it's feminine.

And yeah, I don't know myself what regional greetings you could have been given. Although one that I'd recommend because of how often I've encountered it, is "Moin". Comes from Low German and it's a kind of "Hi"... Also the Wiktionary article for "tschüs" (for some reason it prefers a single ending "s") is quite interesting. Well, it's not of High German origin, of course, but apparently it's related to the French adieu (I don't know how familiar you are with that, but usually it's among a few internationally famous parts of French, after croissant and baguette).