This is from the Pub 100-04 Medicare claims processing Manual. How I understand it with the GA modifier is that the provider and patient understand that it will be denied as non-covered because it's not reasonable or necessary but that you have an ABN on file. The ABN must be on file signed by the patient prior to the services being billed to Medicare. The patient must know the exact amount of the cost and this should be reflected on the ABN.
The GA modifier must be used when physicians, practitioners, or suppliers want to indicate that they expect that Medicare will deny a service as not reasonable and necessary and they do have on file an ABN signed by the beneficiary. (See
http://www.cms.hhs.gov/medlearn/refabn.asp for additional information on use of the GA modifier and ABNs.)