Here's what the GA POST web page states:
"With the implementation of this rule, retired officers may elect to comply with the provisions of POST rule 464-5-.03.1 and thus meet the provisions of USC 18-44-926C (c)(5) which states that "during the most recent 12-month period, has met, at the expense of the individual, the State's standards for training and qualification for active law enforcement officers to carry firearms." Please note that there are other provisions that a retired officer must meet in order to comply with this code section, such as: years of service, identification, etc. Since H.R. 218 is federal legislation, it is not an issue directly addressed by POST rule 464-5-.03.1. Thus, individuals seeking authorization under that provision of federal law should acquaint themselves with the law and its requirements."
Under the rule cited above, the instructor must be a POST certified firearms instructor; however, as the quoted passage states, the federal legislation is not directly addressed by GA POST rules.
GA POST has developed this form which may be submitted to receive certification of qualification.