I've been trying to figure this out myself and as far as I can tell there are no easy options since there aren't any HND-GMP flights and the flights getting into NRT all arrive too late to take a same-day connection to ICN. Only workable option seemed to be to add some other stop in between Tokyo and Seoul (Singapore for example) but then I was getting into like a three-day-long flight which makes two-day windows for pre-arrival PCR tests in Korea a challenge.