A person or company that violated the GPL would void the permission granted to them by violating the license and would be liable for having done so. But it would take a court ruling to decide what that means for the parties involved.

For example, the court might give the violating party the choice to either release source, OR to keep the source closed but to financially compensate the copyright holder(s) of the code that they violated the license for. For example. Another possible outcome could be that they could say that the violating party is given the choice between releasing the source OR recall the product and financially compensate the copyright holder(s) of the code that they violated the license for.

Like I said though, IANAL.