Originally posted by C0untzer0:http://www.blender.org/education-help/faq/gpl-for-artists/seems to answer your question more fully than anybody here can.
Originally posted by http://www.blender.org/education-help/faq/gpl-for-artists/:So I can make games without having to worry about the GPL, right?Games created in Blender (.blend files) are program output and therefore not covered by the GPL. You can consider them your property, and license or sell them freely.With stand-alone games however, any data that is included inside the actual stand-alone executable is covered by the GPL. If this is a problem then you should set up the stand-alone player so it reads from external .blend files. A common practice is to include a "please wait, loading..." scene in the standalone, and read level files externally.The Blender standalone player or the game player can be sold commercially too, but you have to make it available under the GPL conditions.
Originally posted by http://www.gnu.org/licenses/gpl-faq.html#GPLRequireSourcePostedPublic:Does the GPL require that source code of modified versions be posted to the public? The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization. But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL. Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.