There are a lot of misunderstandings about what the GPL actually is and actually means, and I'm detecting one in your question:
everything you make with the engine is forced under GPL
That's actually not the case at all. The engine source remains under the GPL for sure, but it pays to have a read of the GPL FAQ, particularly the parts of it that relate to the output of a GPL program, which I believe are relevant here: http://www.gnu.org/licenses/gpl-faq.html#GPLOutput
Is there some way that I can GPL the output people get from use of my program? For example, if my program is used to develop hardware designs, can I require that these designs must be free?
In general this is legally impossible; copyright law does not give you any say in the use of the output people make from their data using your program. If the user uses your program to enter or convert his own data, the copyright on the output belongs to him, not you. More generally, when a program translates its input into some other form, the copyright status of the output inherits that of the input it was generated from.
So the only way you have a say in the use of the output is if substantial parts of the output are copied (more or less) from text in your program. For instance, part of the output of Bison (see above) would be covered by the GNU GPL, if we had not made an exception in this specific case.
You could artificially make a program copy certain text into its output even if there is no technical reason to do so. But if that copied text serves no practical purpose, the user could simply delete that text from the output and use only the rest. Then he would not have to obey the conditions on redistribution of the copied text.
This changes your situation quite a bit. You're not actually in a position where any content you develop for or with this engine also falls under the GPL; that content is yours, you own the copyright for it, and you can license it as you wish.
IANAL, etc
