Since "γεια σου" is used as both a greeting and a farewell, could this not also mean 'Goodbye and thank you"?
Yes, it could also mean that, we just tend to translate goodbye as αντίο, but you are totally correct. Γειά σου means literally "health to you" so it is used for both hello and bye! :)
Ohh, that's something new to me. Didn't know γειά σού is also used as bye, thought just as "hello". xD
Well thanks for sharing that, I appreciate it. :]