So far, all additional content has been available through free downloaded patches (e.g. Beach Bum, seasonal items, V-Day Massacre), and not through disc-based content to purchase separately like TLaD and TBoGT, if that's what you're referring to.
In some cases the content itself is free in-game (e.g., Beach Bum clothing was all free), while V-Day Massacre clothing, weapons and the Roosevelt had to be purchased in-game. These new items will have to be 'purchased' in-game as well, according to the Newswire article...
Just like the recent Valentine's Day Massacre Special, the new vehicles and weapons will be available in both Story Mode and Online. In Story Mode, weapons will be deposited into all three characters' inventory with two full ammo-clips and all three cars are accessible from their garage properties. In GTA Online, you'll be able to purchase the new weapons and their attachments at any Ammu-Nation location while the vehicles will be available for purchase from Legendary Motorsport and Elitas.