3Dmagix facts

Slightly off topic but,

First, you do not need to publish your blend files if you do not want to. Second (if you do release your blend files), they are not automatically licensed under the GPL. Blend files are your sol property and you may release them without using the GPL if you wish.

If you release your work under the GPL, then yes, it would be legal (but it’s very unethical).

Unless stated otherwise (and I never bothered to check since I don’t use the game engine), your game wouldn’t get “infected” by GPL. You’re not extending or using part of blender. You’re using it to create something thought blender’s features.

As far as I know your game does get affected by the GPL if you use the Blender Game Player when turning it into an .exe. That’s why there’s the BPPlayer, Erwin’s .blend file reader, ect…