License a project under the GPLv3, but license a library to communicate with it under the MIT. Is it possible?

Question

I am currently writing a game. Leaving aside all the game development things, I want to create a library implementation for its API, basically so I give some random guy on the internet the ability to modify the game without writing source code for it. Currently, the entire game is licensed under the GPLv3 (or later), however, I want the library to use the MIT or another permissive license, so not everybody has to comply with the GPL license to write a simple plugin for it. There are three questions I have:

• Is that even doable under the same project?
• If not, can I write the library seperately, and then make the game link into it always? If yes, does it need to be on a seperate repository, folder or not associated at all with the game?
• Assuming 2 is a yes, are there any catches?

Keep in mind this library is pretty much mandatory for the game to work.

Answer 1

From your comments I understand that the game and the plugin(s) will run in the same process. In that case, the GPL license of the game also exerts its influence on the plugins, regardless of how permissive you make the license on the interfacing library.

To achieve your goal of keeping the game open-source, while allowing closed-source plugins, you can add a special permission to the license of your game. Something along the lines of

As an additional permission, plugins that link to through its plugin interface are not required to comply with the requirements that the GPL license would otherwise put on them.

Having a separate license on the interfacing library is technically possible, but I would advise against it when the rest of the project is under a strong copyleft license, like the GPL. The reason is because the separate license only really becomes effective when the code gets copied to another, independent, project.