I think the 90 day residency only applies if you are not a US Citizen. If you are a citizen and you have established residence in Georgia you can apply right away.
It is really easy to get in Gwinnett County. The license is good for 5 years. No training is required. Simply fill out the application without signing it, go to the probate court in Lawrenceville (I believe it is "Vital Records"), give them some money, raise your right hand and then sign (when...