Who qualifies
Two conditions do the work. The licence must be current rather than merely once held, and it must have been earned by examination. The reciprocity application itself is available from the Forms and Applications section of the GREC website.
The Florida exception
Florida residents who qualify for reciprocity cannot use the standard application. They must take and pass the Georgia Supplement Exam, applying through PSI using the same procedure as any other candidate. If that is you, the exam guide and the Georgia differences ledger cover the state material the supplement tests.
Confirm your own eligibility
GREC decides eligibility, and the details of an individual licence history matter. Contact them before paying for anything, rather than working from a summary on any website including this one.