I know that under the GPL I have to release my source code and that anybody downloading my game has access to the source code and can redistribute it freely. However do I still have rights over my models, even though they are bundled with blender player. Quake for instance, it is open source and under the GPL, but the models/textures/sounds aren’t. Thus making it illegal to reproduce and distribute a copy of quake.
I think that you realistically have to assume that “model data” is going to be accessible. It’s your choice alone whether you want to distribute your game open-source. Bear in mind that, either way you do it, the model files will probably still be extractable by someone who is actually determined enough to do it. You need to decide for yourself just how likely that is, and just how damaging you consider it to be if it happens. If you don’t like the idea, don’t use a GPL license. (It’s your copyright.)
I think it depends on your own thoughts! If you want to share it with them to get more feedback! Then that is OK!