Sorry for the delay, I know you've been trying to get an answer for a while. :sweat:
I make a lot of free software (libgdx, Kryo, KryoNet, etc) but, as you found, the Spine runtimes license requires a valid Spine license. It isn't be possible to release the Spine runtimes or derivatives under a GPL license, as that would discard the Spine runtimes license. The intent is to prevent competing tools from leveraging the Spine runtimes, which were developed for use with Spine.
This causes an issue with OSS and other software projects that don't want to be encumbered by the Spine runtimes license. The usual solution is to make the Spine runtimes portion of the software project an optional module/plugin/library which is licensed separately under the Spine runtimes license. You may even have users go and grab the Spine runtimes from the official GitHub repository themselves. Either way, this approach gives users of your game engine the choice to opt in to the Spine runtimes license if they want, while still allowing you to license the rest of your game engine how you like.
