The DLC plan for Grand Theft Auto V has been leaked via one of the in-game screen. As per the details in the image below, GTA V DLC will be available in the form of a special weapons.
These weapons will be made available on Xbox Live game store. You can check out the image below it showcase plenty of placeholder space at Ammunation.
The description under Ammunation Menu (as seen in the image above) reads something like this, “Visit the Xbox Games Store to find new downloadable content”. The second screenshot further details the DLC contents.