The Confucian, Hayashi Razan who served Tokugawa shoguns in Edo-era highly praised Gero Onsen for its quality. Since He wrote about Gero is the best of all Onsen in Japan in his poetry, Gero Onsen has been one of the three Onsen of Japan.
One of Japan's Three Famous Springs, not to be confused with Japan's Three Great Springs and several other competing variants, The town's hot springs made their first appearance in print in the Engi-era (901-923).
As you'd expect in a hot spring town, the major attraction in Gero is to soak in hot springs. An easy way to sample a number of them is to buy the Yu-meguri Tegata pass, a wooden amulet sold all over Gero. This will get you into 3 hot springs of your choice from a choice of over 20 for the flat price of about ¥1200, which can be used up at your own pace as you'll get a stamp each time you visit.
Foot baths (ashiyu) are popular in Gero, as the waters are reputed to have curative properties. True to the name, you just take off your shoes and sit down with your feet in the water. A number of free foot baths can be found around town, including the decidedly tacky Venus Foot Bath in front of the Shirasagi Hotel — no prizes for guessing what the statue in the middle is doing.
A giant hot water pool (notenburo) can be found to the west side of the bridge, with access via the south side. Note that there are no facilities to speak of and the bathing area is mixed and clearly visible to passersby on the bridge above, but admission is also free so you get what you pay for.