I made a game, and I am currently making a game engine. I want them both to be completely free and open source. What license should I choose? I was reading a bit on GPL, but that seems to be more suited for system code and libraries, AFAIK, as it doesn't permit the use of code for proprietorial software - which, in turn, implies that the code can be used in the first place. I can see that, obviously, game engines can be considered libraries, and therefor be used, but what about game code?

Is there an alternative to GPL?