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. You should be able to delete the campaign if you don't want to buy it.

If you will be playing the game campaign for the game no need for an Xbox Live Gold, you just need to set your console to offline. To do so please follow the steps below: Settings > General > Network Settings > Go Offline.