GPL is pretty clear cut. You derive from a GPL-licensed work, that derived work is then licensed under GPL. You use a library, then that library must be GPL compliant. There's loopholes and technicalities but you aren't going to be able to do that in this case. As far as assets are concerned, you are not allowed to redistribute assets from the original game with code. This is considered an intellectual property violation/piracy. The exceptions are the things that were distributed with the original code release. I believe this was a few .cfg files. As far as pay, yes, you can charge for productions which use this code. This adds one extra restriction: you may not use copyrighted content (such as trademarked stuff, stuff protected under copyright, etc), including any mention of Star Wars stuff. Regardless of whatever path you take (paid content or free download) you must provide either a link to the source code, or package the source code with the finished product. If its a paid product, you can choose to make the code only available to those who have purchased the software. This is optional; Quake 3 and others make the code available for free. It's pretty clear-cut.