Treat the cave as a planned visit, not a quick sheltered stop: the public route is uneven, damp in places and rises between levels.
Opening and admission
Korea Tourism Organization lists Gosu Cave as open year-round, 09:00–17:00. Its listed admission is KRW 11,000 for adults, KRW 7,000 for teenagers and KRW 5,000 for children. A Korean Heritage Routes page gives 09:00–18:00 with last entry at 17:00. To avoid arriving after entry closes, plan to be there before 17:00 and check the cave operator for the date you visit.
What the walk involves
The heritage route describes about 940 metres of the roughly 1.4-kilometre cave as open to visitors, with around 30 metres of elevation change. Expect steps, low light and damp surfaces; it is not a level indoor walk. The route is listed at 8 Gosudonggul-gil, Danyang-eup. Use the KTO listing for current visitor details and the Korean Heritage Routes description for cave-route context.
If anyone is uncomfortable with confined or uneven spaces, make the riverbank or a town stop the shared plan instead. Keep the cave as the main activity and add only one easy stop afterward.