As your departure approaches, be sure to check South Korea's entry rules and the sleeping in airports website since there seems to be a significant chance of a missed connection (which could also complicate COVID-19 testing requirements for your destination, if applicable).

Is there any chance that there's a same day flight from Gimpo?

You could ask DL to change you to a different TAPC flight that would arrive into ICN earlier even though it's likely to be an illegal routing.

Have you checked the inbound flight? If the turnaround time is tight, avoiding Russian airspace flying east across the Pacific (from ICN, HND, etc. to SEA) could explain departure delays.