No, it's not breaking the GPL. You don't have to publicly release the source at any point in time according to the GPL, but what you do have to do is offer to give it to anyone who asks for it (optionally at a cost).
In the case of a mod, provided they haven't also modified the engine, they don't have to do anything. Just point anyone who downloads it at the original distribution and that's all that's needed to satisfy the GPL.