ICN vs GMP?
 

Flights from Tokyo are close-enough in price between ICN and GMP.
I'll be flying either Asiana or ANA, to use *G status.

Seems like you would fly to GMP if you want a shorter commute, and don't care about the airport experience (no shopping, no restaurants, crappy lounges).

Is the extra time to get to ICN worth it, or is GMP a no-brainer?

I'd go for GMP, its a much quicker commute and is easily accessible by underground.

Frequent user of both airports, prefer GMP by a huge margin. Checkin to gate in no time also in the international termninal, tax free selection is surprisingly good due to demanding Japanese travellers and the landside 2nd floor Cafeteria is actually one of my airport favourites (abalone porridge and other dishes)

Commute times are also more human than to ICN.

Is that the amusingly named Sand and Food?

Sorry for the primitiive description: Coming from the check-in aisles turn right before security and take escalator up one floor. The establishment I frequent is to the right on the upper floor.

If arriving, GMP for sure! No queues for foreigner immigration unlike ICN. But I guess you also have to think about the TYO end as well, because NRT is even more of a pain to get to than ICN.

It all depends on your travel plans. Are you connecting to another flight or staying in Korea for a bit. If you're just passing through, the best option would be HND-GMP.

My inbound flight from NRT to ICN was late and I missed the next KAL bus was going to be a 45 minute wait... Based on the loooooonnnnggggg train ride into the city, I wished I had flown into GMP just to shave off 20-30 minutes of train ride.

Immigration in the 10-11p hour at ICN has essentially no line. It took me more time to walk from the plane than to transit the immigration line.

Spending 5 days in Seoul this March. Will fly Tokyo-Seoul

Assuming that GMP is still the preferred arrival airport based on above comments?

Definitely! You will save at least 30 minutes in transit time, and at least the same in airport time.
Gimpo is such a pleasure to fly in and out. Furthermore, you will depart from Haneda, which is also
much closer to Tokyo than Narita.

Thanks for the feedback, I'm excited to try two new airports for me as well, Haneda and Gimpo

20+ years ago I commuted regularly between HND and GMP. GMP international was this small but pristine terminal that was pretty much deserted, as the only flights in and out were to TPE and HND. HND international was literally a hangar shack with two (or three?) gates. Roll to the present, and GMP international is this small but pristine terminal that is pretty much deserted, as the only flights in and out are to TPE and HND. HND international is now this massive terminal with dozens of gates and heaving with people all the time. I still love GMP, and HND although an excellent airport, it no longer has "insider" charm.

And yes, the little restaurant/cafeteria past security is really excellent. The lounge is meh, but cute.

While you are correct about the small but pristine terminal, I have to correct the rest of your assertion.
There are flights to TSA, not TPE ( and it has always been that way).
There are also flights to KIX, PEK, PKX, SHA, and KHH ( although that last one may only be a couple of times a week.
It is not that deserted any more.

Is this restaurant/cefeteria at Haneda or Gimpo?

GMP, where there is only one restaurant. HND is another matter. There are dozens of retail and food outlets past security.