There are a lot of ways to say "goodbye". I mean you don't usually say "goodbye" to someone you're talking to in English too. It's really formal. In Greece we use "bye" A LOT but we also use "Τα λέμε" meaning like "cya" and we also use "cya" here. But mostly used is the same word you use to greet someone, the word "γεια". It can also be used to say goodbye to someone. There are a lot of ways but i think if you ll be talking to a friend or relative you can use "bye" or "γεια"
I'd also like to point out something a lot of people often overlook, but I use it all the time. The phrase "Γεια χαρά". On its own, Γεια means health, sticking χαρά next to it adds the word joy. Γεια χαρά, health and joy, a greeting as well as a goodbye, this is informal. If you want things to be formal, then you just need to say Γεια σας, which is a plural of formality