HI, HELLO = Generic greeting
GM = Good morning
GA = Good afternoon
GE = Good evening
GM/GE = This was used in the QSO example. It refers to a QSO at noon UTC which would be in the morning in New England and in the evening in East China. Such a split greeting is not uncommon in inter-continental QSOs.
GD = Good day (This is a farewell greeting, do not use at the beginning of a qso unless you are talking to down under)
GN = Good night (do not use in the first part of a qso. This is a farewell greeting).
GB = Good bye (This is a generic farewell greeting)