That's what bugs me about that one.

Saying "good morning" or "good evening" or whatever is a salutation. It's not conveying any real information, it's just ritual politeness. I'm not a big fan of ritual politeness. But, if you're going to do it, don't half-ass it. To me, it's impolite and insincere to half-ass or abbreviate a greeting.

I feel the same way about "GG" and "TX" for thanks. Do it right, or don't do it at all.