Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 19:55 and arrives at Tokyo Haneda Airport at 22:15.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 621 Km.
The best month to arrive at Seoul is March and the most expensive month to fly is July. (Average prices, based on 857 datapoints.)
We have a lot more statistics for flights GA 9104 and between Seoul and Tokyo.