Korean greetings, good morning!

I don't think "good morning" is a traditional greeting in Korean. However a lot of koreans are already familiar with Emglish. So some people use this exprrsstions in Korean too.

Normally you can use this at your work/school to your collagues/classmates.
However we rarely use this to the boss or teachers.

For your infotmation, we almost never use "good afternoon or good evening".
I never heard "좋은 오후 or 좋은 저녁".
Of course, in grammar is okay, though.