There is NOTHING in the GCA specific to carrying a firearm in a vehicle.
I know what I know based on experience and interaction with local PD.
The question is best posed to the Attorney General, which was actually answered by the attorney general's office during the GGOA public forum held at the Hilton last year.
Basically, If carrying/transporting a firearm in a vehicle, the person in "possession" of the firearm is required to have a concealed firearm I.D. To be considered by LEO "NOT IN POSSESSION" the firearm is required to be stored in a locked container with the ammunition separate from the firearm in an area of the vehicle not accessible to ANY of the occupants in the vehicle.