The traffic around GMP can sometimes be a bit heavy, but if you have an 0800 flight (I assume it's international?), you'll probably want to arrive at least 90 minutes before to be safe. At that time (0630) traffic likely won't be much of a problem.

Likewise, if you're not overloaded with luggage and want to take the subway, it shouldn't be too crowded at that time, especially going in that direction. The number 5 line goes to GMP, so if you're staying nearby a station on that line it would be pretty convenient. There's also a new number 9 line that goes to GMP, with express trains, but that's from the southern part of Seoul (Gangnam area), not the center, so likely of no value. With the subway, it'll take a fixed time to the airport so no need to worry about traffic.

Taking a noon flight out of ICN means you'll hit a lot more traffic if going by road, though after you get to the tollway (in the Gimpo area), it's clean sailing the rest of the way to ICN.

If you take the subway to ICN, you have to transfer to the AREX train at GMP. The AREX shouldn't be crowded, but you'll hit the subway during a fairly peak time, so not a lot of fun together with any bags you have. The AREX section from Seoul Station isn't set to open till I think around the end of this year, which at that point taking an express AREX train directly to ICN would be the way to go.

If you're an early bird, and it's very important that you make your flight, I'd probably opt for the GMP flight myself, just because I'd have to deal with less traffic or busy subways. ICN is about 30 minutes more of travel time, but with traffic would probably be an hour+ more.

If you have lounge access, I'm not sure what the KE lounge is like at GMP, I've only used the OZ one, which is nice, though not as good as at ICN and without showers. I doubt the KE lounge at GMP has showers either. For a flight late in the day, after working all day, and not being able to shower before heading to the airport, I'd choose ICN where I can get a shower in the lounge before my flight. But for a morning or noon flight, I wouldn't care about that.

Whenever I've taken a morning flight out of GMP, the immigration lines are very short, much shorter than at ICN. The lounge at GMP however I believe is landside, so you end up needing to allow a bit more time in case you get delayed at immigration.