I have been part of both a "Trade Allowance", and a full "Buy Back".
They will give you fair market value for the truck, with a few deciding factors how they come with the figure. I would ask for specifics and details in writing before I pulled the trigger.
I came out in better shape both times on how much I owed, and total for monthly payments. If you do trade yours in on a new GM product, you can use all current rebates, and any incentives available (I was able to), so you can get a pretty sweat deal on the overall trade.