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.
I don't think this is entirely correct. It's only protected by US law in the US - in all other countries, it's protected by their local laws. That's the reason for international agreements to harmonize copyright between countries, such as the Berne convention.
I said the work was protected by US copyright law, not that US copyright law would necessarily be used in the case. That the opera is a US work can be relevant to jurisdiction rather than the case in chief.
Copyright would probably not be an issue. This looks like a simple contract dispute, a debate re whether one party is abiding a term in a licensing agreement.
If EU law is applied, no. If American law control, probably yes. This is a contract between an american and an EU organization so there will be a fight over which law controls the contract. Due to the split between the various US states, American contracts normally have some language stating which jurisdiction's laws should be used to interpret the contract. If this was a stock licensing deal from the estate it probably names a US state.
