This guide will let you know how to move the cars from one garage to another in GTA 5 Online.
When you are outside of your garage, you have to hold up on the d-pad and then use your game phone. You have to go to your contacts list and call your mechanic.
After that, select your current location and then select the car you want to deliver to the other garage.
Then you will need to wait for a few seconds, and you will see that your car has been delivered to you. You have to sit in it and drive it inside your garage, as shown in the image below.
The next thing you have to do is dial the mechanic again, select the car for delivery, and then select your current location of the garage. You have to wait for the delivery, and the delivery time will be about 45 seconds this time. When you ask for more car deliveries, the delivery time will also increase accordingly.