Gershwin was american. I assume his kids/estate is american. So this involved a contract between a European opera company and US persons regarding a work protected by US copyright law. They could probably file in a US court and a court may well determine that the US is the better place the hear the case. (When between a person and a corporation, US court tend side with the person's location on issues of jurisdiction. If someone is going to get on a plane, they think it easier for the corporation.) It is a possibility.