The GSA was a compromise between WMI which filed for BANKRUPTCY and the FDIC which seized WMB and placed it in RECEIVERSHIP.
The FDIC is bound by that agreement which they signed onto to settle outstanding disputes with the Debtors, unlike the bankruptcy court which had limited power, and in the case of the P&AA with JPM, ZERO jurisdiction.
That's why...or maybe, according to your logic, the FDIC signed another GSA we don't know about.