Call of Duty Warzone is a free to play game. It's not on GamePass and it doesn't need Xbox Live Gold either. MW2 and its multiplayer component is not free. You need to buy it if you want to play the single player game or MW2 multiplayer.

Call of Duty: Warzone!" Free for everyone. Drop in, armor up, loot for rewards and battle your way to the top.

Call of Duty 4: Modern Warfare (2007) on Steam. The new action-thriller from the award-winning team at Infinity Ward, the creators of the Call of Duty series, delivers the most intense and cinematic action experience ever.