Have you read the GNU license information? If not, I would highly suggest you do so. It explains what must be covered by the GNU license. In short, your game does not have to be covered by the same license. The engine your game is using does. In other words, if you add any code that does not in any way alter the engine, that code is yours and you can license it as you please. That also goes for all assets (art, music, even the event system you built in the engine). You own the rights to your creations. If however, you do alter the engine some way, you must release those changes to the community. Also, you are legally obligated to include the license information for the engine in any derivative product you release.
As for making a free game, yes you can as long as you adhere to the license.
Finally, I believe the source code is found here:
I hope that gets you what you need and good luck with your project.