As I thought.werdanith wrote: If you take OpenMW, make changes to the source code, compile it and ship it with your game assets without making the changes available then yes you are breaking the license.
GPL is good choice for engine reimplementation, but not suitable for further development of OpenMW as game engine. Moreover, almost all open-source game engines uses permissive licences (Irrlicht, Panda3D, jMonkeyEngine, Torque3D, Nebula Device) or LGPL (Delta3D, Crystal Space).
It would be necessary to think about changing the license in future, after OpenMW 1.0.