 On 10 April 2016 the former municipalities of Vergeletto, Gresso, Mosogno and Isorno merged into the municipality of Onsernone.The discovery of a Roman tomb from the 1st-2nd century AD, with pottery and coins, proves that Gresso was part of the outer-most reaches of the Vicus of Muralto.The modern village of Gresso is first mentioned in 1316 as "Agrassio".