The easiest thing to do is to purchase a GSX from a dealer (this isn't going to be the cheapest).
Here is how it normally works:
1. You owe more on the car than it is worth.
2. The dealer takes the difference between what you owe on the car, and what they will give you in trade and adds that to the price of the car you are buying. They call this "rollover".
The amount you can rollover is dependant on the age of the car you are buying, price and how much the dealer is making on the car (the one you are buying).
Normally this is EASY when you are buying a new car, but much more difficult when buying a used car.
I would find out what the car is worth (
www.kbb.com) for a private sale and find out how much you owe to the bank. Then take out an ad somewhere (paper, online, etc) asking for the difference (between what you owe and blue book) and to take over your payments.