OCGA 16-11-126(a)
Your answer is right there....
You can carry it loaded in your vehicle once you are 18 in the state of Georgia. Period.
I had my criminal justice professor, who is also a Gwinnett county judge, confirm that for me.
Don't let all the "unloaded in a case" "magazine out" "3 step rule" confuse you. You can legally carry your loaded handgun in your vehicle.
With that said, there are quite a large number of LEO's that do not know that. And you could run into problems with them.
Personally, I don't let that deter me from doing so. But that is just me.
Winner winner chicken dinner