The Center for Medicare and Medicaid Services (CMS) created two modifiers that allows you to distinguish between services that are statutorily not covered or otherwise not a Medicare benefit because Medicare does not consider them “reasonable and necessary”.
Modifier -GY: Appending -GY modifier to the CPT code enables one to 
identify an “item or service is statutorily excluded or the service does not meet the definition of Medicare Benefit”. This will automatically create a denial and beneficiary may be liable for all charges whether personally or through other insurance, ( for example: when a beneficiary wants new eye glasses and wants to get a denial through Medicare for secondary payer purposes), claim should be submitted with -GY modifier. This way claim may be processed faster than it would be without -GY modifier. ABN’s ( Advanced Beneficiary Notices) are not an issue for statutory exclusions.
Modifier -GZ: You should append -GZ modifiers to CPT codes when you think a service will be denied because it does not meet Medicare policy standards for medically necessary care and you didn’t get an ABN or patient refused to sign an ABN and you nevertheless, did furnish the sevices. By using -GZ modifier, you are notifying Medicare that you know an ABN should have been signed but was not and that you recognize you made an error. This modifier is a measure of good faith towards Medicare. Note: You cannot bill patient for these services.