@GamerTurtle Unfortunately that's not possible. Stadia is its own platform, so you need to by games from Stadia store to play the game on the platform. It's the same concept as Epic Store and Steam for example. In order to play the same game through both platforms, you need to purchase the game from each storefront.
Man that would be a killer feature where we essentially are paying for the cloud play feature of stadia and import the games that are on both for some small fee or conversion fee (even if essentially it is paying again for it but at some discount).
I would pay for that and it would be a huge feature that other platforms that do not have these capabilities