http://hn.vernccvbvyi5qhfzyqengccj7lkove6bjot2xhh5kajhwvidqafczrad.onion/stories/33962042
So you can release the source for your game under GPLv3, you just can't distribute the engine alongside the source. Someone would have to own the engine and use your source alongside it to contribute to your game. You can compile and sell your game as you normally would, of course!