If you already own a copy of a game on XBOX GamePass, you can still play it. I learned this with Skyrim Special Edition. If you own the game, digitally or physically, your ownership of it overrides the need for a GamePass subscription. You can still play the game because you (or the person who gave it to you) bought it with your (or their) own money, and so they legally own it. The GamePass versions of games are technically different from physical copies or digital copies acquired before it was added, so you can still play it if you bought it prior to it being added.
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