ressurection666

Best answer as chosen by Asker / Ratings
In order to impound gang cars you simply need to get in the car and drive it into the agency tower and store it in the garage there it will then be stored for you to collect at any time you wish. You can also check which cars you have not already impounded at the garage so you know which ones to look for, there is an achievement for impounding all the cars.

Hope this helps