Neither. Use GY (known to be not a covered benefit) vs. GA or GZ (expected to be not reasonable and necessary but would otherwise be a covered benefit).

GZ Modifier: Use this modifier when the claim is expected to be denied as "not reasonable and necessary" but no ABN was obtained.

GA Modifier: Use this modifier when the claim is expected to be denied as "not reasonable and necessary" and the ABN was signed and is on file. You should also use this modifier when the beneficiary refuses to sign the ABN but still demands the service if two witnesses have signed the ABN form noting the patient's refusal to sign.

GY Modifier: Use this modifier when the claim is expected to be denied as "non-covered". This modifier is for informational use by the Carrier and does not prevent directly billing the patient for the service.