ごきげんよー ＝ ごきげんよう（ご機嫌よう）
This word is an abbreviation of "ご機嫌よくお過ごし下さい".
"Please spend some time in a good condition," or
"Please sepnd some time in a good mood".
Since it has been saying "お過ごし下さい(please spend some time)", it sounds more like saying "good bye/have a nice day;" but, this expression could be used as saying both "hello" and "bye".
If my recognition is correct, this expression was originally used by only those very royal people such as Japanese King and it's relatives; however, now days, it became so popular that anyone could use it.
Thus, you could use it instead of saying "hello" or "bye" in a daily conversation.