I've done a fair bit of open source work, but the best I could come up with for games is "source available" where you can get the code to recompile for yourself or mod it. (My wife and I put up the source code of our first game as a DLC on Steam as an experiment.)
Re only code: Game engines often tie code and data together and the code without the data won't even compile in many cases, or you won't be able to produce a new dataset because documenting the required settings is a monumental task.
Finally, there is no real benefit to open sourcing a game if you want to make money. There is no complementary product or service you can sell with it. Reviewing and incorporating community fixes requires work and potentially legal review as not everyone in the modding community is also necessarily well versed in IP law. All these things need funding which is not a given if the game is open source in any meaningful way. If a game is truly open source, a less than ethical company with a bunch of cheap labor can go and undercut you, so there is an incentive to make creating an alternative build as hard as possible, defeating the entire idea of open source.
I would like to be able to release on an alternative storefront so people who want to degoogle their phones, but F-Droid's policies (most known alternative) make that impossible. (To be clear, it's a perfectly legit choice, I just wish there was a meaningful alternative to Google.)