Can I leave ICN airport on an 8-hour layover? Indonesian citizen, US Green Card

As your linked site confirms, as an Indonesian passport holder and an individual with US permanent residency, the Republic of Korea allows you to travel to or transit through Korea without a visa. You will be able to stay for up to 30 days in Korea.

Tourists in transit to Third Countries

1. Eligible Applicants
   Citizens of all countries except Syria, Sudan, Iran, Macedonia, Cuba, Republic Of Kosovo, Palestine, Afghanistan, Iraq, Nigeria, Ghana, Yemen.

2. Requirements
   Persons who hold a visa (re-entry permit, permanent residency, etc.) to enter the United States, Canada, Australia, or New Zealand (hereinafter referred to as “the four advanced countries”) and
   ? who go to one of the four advanced countries in transit through Korea; or
   ? who stay in one of the four advanced countries and take a direct flight from the country to Korea to go to a country of nationality or a third country.

Please be noted that:
? If you are a U.S. visa holder who transit through Korea and Saipan to reach the U.S. as the final destination, you are permitted to enter Korea without a visa. (However, if you depart from the U.S. and enter Korea via Saipan, you are not granted no-visa entry to Korea.)