These are my notes, but I haven't updated them in a very long time, because we rarely provide a service that requires an ABN. I believe they are still correct.
GX: used to show that you are providing a service that is never covered, and you DID get an ABN.
GY: used to show that you are providing a service that is never covered, and you DID NOT get an ABN.
GA: used to show you expect that Medicare will deny a service as not reasonable and necessary, and you DID get an ABN.
GZ: used to show you expect that Medicare will deny a service as not reasonable and necessary, and you DID NOT get an ABN.
I recommend never getting yourself into a GY situation. There is no reason to provide a service that you know is not covered and NOT have the patient sign the ABN. It's a CYA situation - the patient is acknowledging Medicare never covers the service, and that they are financially responsible for it. Otherwise, all they have to do is call Medicare and complain that you did not tell them they would have to pay for it, blah blah blah. Just don't go there.