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G codes are part of the HCPCS series codes that you can use (mainly) for Medicare providers. They are in a book similar to the CPT and ICD-9 books.
If you go to: http://www.cms.hhs.gov/MedHCPCSGenInfo/
it may help you understand it a little more.
I still have issues with them. Not only are the "G codes" but there are A codes, J codes, S codes, etc. It's neverending.