Gero Onsen is the town's reason for being. The alkaline water is known for leaving skin feeling smooth, and the town makes bathing easy even if you are not staying in a ryokan: public baths, hotel day-use baths, and free foot baths sit within a short walk of the station and river.
Do not treat it like a quick photo stop. Gero works best when you slow down, bathe, walk the river, eat, and bathe again.