There's a project called Endless Sky
It's license for Source code is GPL,
The license for Graphics is CC-BY, CC-BY-SA and Public Domain.
The license for Data .txt files with content like this is also GPL
outfit "Small Collector Module" category "Power" licenses Coalition cost 60000 thumbnail "outfit/small collector module" "mass" 8
on offer conversation `When you repair the <origin>, the captain hails you. "This is the fourth time I've nearly died in the past week doing this job! Here, take this <cargo> to <destination> and finish this job for me. They said the payment is <payment>."`
Data files define all the ships and such but it doesnt call any APIs, and is not linked. It's like a high-level scripting language for a VM. It's not neccessary for the game engine itself to exist. It describes gameplay (ship statistics), quests and the universe(where planets and systems are and how they connect)
Graphics are in CC-* or Public Domain.
My research suggests that:
- GPL source code is infectious, so I'd have to write the engine from scratch
- GPL data is infectious so I need to release all data changes (except for mods) in GPL
- CC-*/Public Domain content (audio/visual) is OK to be used where I can't provide the source code.
Is creating an open-source engine for Endless Sky in Unity that uses the same data and images and sounds, but is unable to release the Unity source code (obviously) be a violation of GPL?
I know there's GODOT and it's MIT, which is compatible with GPL, but I'm asking because I would like to understand is, if GPL game content "infects" the game engine with GPL or not.
I would also like to know if creating non-GPL mods to a content that's GPL is possible or a violation of GPL as the data is being merged together(a mod might take some GPL-covered ship statistic and modify it, like price, shields or armor amounts).