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Engine released under GPL, but the media required to play the game is commercially licensed.

It's what John Carmack did with id games.

You pay the game creators for their art, or you can find free art and play the game with that.

They could sell CDs with it on. They could sell T-shirts etc. They could sell fancy graphics / avatars etc.

In todays market, selling it through Steam/GOG and co could work too. Enough people might do that just for convenience sake, even if they could easily download a binary release for free.

It's very similar to what OsmAnd (free software map application for Android is doing).

The standard application from the Google Play store is limited in how many maps you can download and you can purchase the OsmAnd+ app for unlimited maps. Or... Get the same OsmAnd+ from f-droid :)

I quite like this model of charging users that are using the proprietary ecosystems. It sure worked on me, I happily paid for OsmAnd+ =)

Who in 2019 is going to buy a CD...?

I think stuff like Patreon works pretty well for supporting open source projects.