Στην υγειά μας / Γειά μας / Στην υγειά σου /Γειά σου
All of the above are common variations of "cheers" in Greek. They are interchangeable and are all derived from the word "ygeia"=health.
Γειά μας = Yia mas = short form of "Στην υγειά μας" which is literally, Health to us, To our health, i.e. we drink to our health. Addressing all the people who are present.
Γειά σου = Yia sou = short form of "Στην υγειά σου", literally, Health to you, We/I drink to your health. Addressing one person.