Where it is mixed, the derivative work would usually be expected to be made available under the terms of the GPL, because the copyright held by the creators of

Where it is mixed, the derivative work would usually be expected to be made available under the terms of the GPL, because the copyright held by the creators of the original work includes the right to create and distribute derivative works. The licensor of the derivative work can refuse to give you a licence unless you agree to some other terms (e.g., money, and/or the right to terminate the business relationship if you exercise your GPL rights), but those terms cannot directly prevent you from exercising your GPL rights, or the licensor would be in violation of the GPL.