Title says it all, here are my concerns.

1. I want a copyleft license, but not at the cost of having to reveal private code in a game project.
2. I mostly just want to create a game engine that can serve as a 'base' that other developers can build distros with that may target different languages, platforms, runtimes etc etc.

I will likely be building my app with GTK and some language bindings, but those are my main concerns. Otherwise, If companies have to release their code with my engine, I would switch to something more permissive like Apache 2.0 If anyone can give me an in depth answer for someone who wants to do exclusively open source, please let me know! Thank you very much ahead of time!

Edit 1: So just to be clear, I am looking at GPL/LGPL but if game developers have to release their source code for their game, I would like an alternative. I am mostly just concerned about the GPL 'share source code' clause with new projects that build off my soon to be work.