Below are some informal ways to say hello and goodbye to each other.
Güle güle is an abbreviation of Güle güle git, which literally means go laughing.
Therefore güle güle is only said to the person(s) leaving.
If you are leaving yourself, you shouldn't use güle güle.
Below are some more formal ways to say hello and goodbye to each other.
Günaydın is only used for saying Hello and the other three can be used for saying both Hello and Goodbye.
For saying Hello the last three alternatives are particularly polite, and most people use Merhaba instead, which is slightly less informal than saying Selam.