@Paul_Crowther is this for a specific duration or indefinitely?
GamePass usually lasts for 1 month+(sure Ive seen them there for 3 months before but could be wrong), not sure of end date but the idea is to temporarily allow people to access the full game so they play it and buy it later on not to 'give it' to them for free. The idea is to get someone hooked on the game. GamePass games usually are slightly newer than the 'free' monthly games for Gold members which are usually the base game, DLC available type or not the latest edition. Using AC as an example the previous game is always on Gold for free when a new game releases, have the old one for free and hopefully buy the new one. GamePass is play the current game for free for a limited time but long enough to make it a habit or that you put some hours in and then cut off access and hope you buy.