According to the insurance regulator of Georgia the state does not have a grace period for car insurance. This means you must have insurance on the vehicle that you buy, either from a dealership or private individual, before you drive away with it. Driving a vehicle that has not yet been placed on your insurance policy, or if it is your first car, not yet had insurance placed on it by you, is against the law and you could be ticketed for driving without insurance.
We have in fact heard from individuals in Georgia that have been cited by police the same day or the day after they bought a car and thought they had time to put coverage on the vehicle but swiftly found out from law enforcement that they did not.
If you already have auto insurance in place on other vehicles you can see if your insurance provider will extend you coverages on a new vehicle you buy for a certain amount of time. Typically though insurers only extend coverages when you are buying a car as a replacement for another vehicle on the policy, for example you traded in one vehicle for the new one, now the new one would be afforded the same coverages as the trade in for a small period of time - usually a few days to a couple of weeks.
If you know that you may be buying a car from a dealer or a private party it is best to investigate what the cost of insuring the car will be and how to get insurance on it as soon as you take possession before you actually go and buy the vehicle. If you have already gotten your "ducks in a row" so to speak with your insurance provider than when you go to buy the car and take possession it may be as easy as calling your agent to give them some details (VIN, etc) and then have your insurance policy in place the day you get the vehicle.
The Georgia Insurance Commissioner recommends that when you get a new car you should keep in the vehicle with you the bill of sale dated within 30 days of the purchase date and a valid insurance declaration page to serve as temporary proof of insurance. They said that you need to be sure to keep the bill of sale and the declaration page in the vehicle at all times until you receive the insurance information card from your insurer.