Güle Güle can be said any time by the one who stays, it's a bit like "have a good trip". Sometimes you see displayed it in Turkey on the road when leaving a province, or leaving the road toll. Iye günler can be said by both, the one who leaves and the one who stays, it means "have a good day"
"Güle güle" is actually an abbreviation of "Güle güle gidin" which literally means "go laughing/go happily". "Güle güle" is, therefore, usually ONLY said to the person(s) leaving. Traditionally, the person leaving would answer "Allahaismarladık". This loosely translates as "with God's blessing" (to express good wishes when parting), but in recent years this word has fallen out of use in favour of "Hoşça kal /Hoşça kalın" meaning "stay pleasant/stay well".
well, Turkish seems to have at least 3 different ways to say goodbye : Gule gule, Hosca kal - hoska kalin. ( excuse me ,my keyboard is in spanish I don't have the umlauts nor the cedilles) ) which of the three is more common ? i would prefer gule gule , it is easier to pronounce. thanks