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.

The base Battle Pass is free to all players. However, upgrading to the full Battle Pass  either as a standalone purchase or included in the BlackCell Battle Pass Bundle  is an optional addition to your Modern Warfare III and Warzone experience.