You do NOT have to be there early AT ALL. The Gimpo International terminal is empty ALL the time, even though all the flights are in banked departures around the same time and the flights are full. There are usually only three or four flights departing around the same time as there are only like five international gates total.
Also, Air China only has that one flight per day, so you don't have to worry about giant check in lines. Literally everyone there is for the same flight, so they will check you all in.
I fly GMP-HND several times a year (usually evening departure, morning return). I arrive only one hour and a few minutes before departure. Check in here for most airlines is 40-60 minutes. There is never anyone waiting in any check in line. The most I waited was for Shanghai Airlines and it was because there were a few in front of me with TONS of luggage.
That being said, it is a ten minute walk from the train platform to the check in area, and I walk fast. So I'd add 15 minutes for a walk/if you get mixed up (you shouldn't, it's very simple). I've walked up to the counter five minutes before closing with no stress or worry.
If your flight is at 9:30, I would aim to arrive at Gimpo Station by 8 with a 10-15 minute walk and you can check in around 8:20 for your 9:30 flight. Security takes LITERALLY seconds and there is NOTHING to do on the gate area.