DISCLAIMER I don't pretend to know anything about licensing. In fact, everything I say below may be completely false!

Backstory:

Recently, I've been looking for a decent game engine, and I think I've found one that I really like, Cafu Engine.

However, they have a dual licensing plan, where everything you make with the engine is forced under GPL, unless you pay for a commercial license. I'm not saying that it's a bad engine, they even say that they are very relaxed about the licensing fees. However, the fact that it even involves the GPL scares me.

So my question is basicly, how does one escape the GPL.

Here's an example: The id Tech engine, also known as the Quake engine, or the Doom engine, was the base for the popular Source engine. However, the id Tech engine has been released under the GPL, and the Source engine is proprietary. Did Valve get a different license? Or did they do something to escape the GPL? Is there a way to escape the GPL? Or, if you use GPL'd source code as a base for another project, are you forced to use the GPL, and make your source code available to the world. Could some random person take the id Tech engine, modify it past the point of recognition, then use it as a proprietary engine for commercial products? Or are they required to make it open source.

One last thing, I generally have no problem what-so-ever with open source. However I feel that open source has it's place, but that is not in the bushiness world.