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.

Modern Warfare 2 Multiplayer Start-Time\n\n Consoles will launch beginning at 4am PT - 9pm PT on October 27th, beginning with regions West of the United States (so New Zealand etc.) Xbox, PlayStation and PC in the United States will all go live at 9pm PT on the 27th, which translates to midnight ET on the 28th.