How much do you pay them?
If the GPL license didn't exist, and instead you just relying on copyright, then that's an injunction. You have to stop using the code you "stole" and pay reparations.
In UK law, if you distribute copyright material in the course of a business you can be facing 10 years in prison and an unlimited fine.
Sure you can't get them to agree to the GPL, they could simply stop distributing and then turn up to their stint in prison and massive fine. In reality I suspect they would take the easy way out and comply with the license.