Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 07:55 and arrives at Tokyo Haneda Airport at 10:00.
The flighttime is about 2h 05m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 696 Km.
The best day to arrive at Seoul is Friday and it would be best to avoid Wednesday, as prices are on average 200.5% higher than on Friday. (Average prices, based on 991773 datapoints.)
We have a lot more statistics for flights JL 90 and between Seoul and Tokyo.