Not sure how I missed this, but if it's correct, it's good news for transiting HND to GMP on oneworld and the first confirmation I've seen aside from the gov't website:
Gimpo Airport to resume international flights in June (koreaherald.com)
Gimpo International Airport in Seoul is resuming the long-stalled operation of international flights from June, starting with popular routes between South Korea and Japan.
According to the Transport Ministry on Thursday, several South Korean and Japanese air carriers such as Korean Air, Asiana Airlines, Japan Airlines and All Nippon Airways have filed applications to resume international flights at the airport.
Gimpo Airport in western Seoul, one of the nation’s main gateways along with Incheon International Airport, is favored among travelers here when it comes to short-haul flights to neighboring countries such as Japan, China and Taiwan.