The family moved to Milan, where Gaetano Belloni was an apprentice in a textile factory. Due to an accident at work lost his thumb and index finger of his right hand, which excluded him from participation as a soldier of the First World War. Belloni began at sports first with Greco- Roman wrestling, but soon came to cycling. In 1912 he made his debut as an amateur. His biggest success in 1920 winning the overall classification of the Giro d' Italia.
Belloni also started at 28 six-day races, of which he won three: 1922 in New York City with Alfred Goullet, 1929 in Chicago with Reggie McNamara and 1930 in New York with Gerard Debaets. In 1932 he retired from cycling, was sports director of various teams and race organizer in Velodromo Maspes - Vigorelli in Milan.
Belloni won numerous races, including
- Giro d' Italia: ( 12th stage, overall winner 1920)
- Milan - Sanremo (1917, 1920)
- Milan -Turin: (1918 )
- Giro di Lombardia: (1915, 1918, 1928)
- Giro della Provincia di Milano: (1917, 1920, 1921, 1922)
- Milan - Modena: (1918, 1921, 1925)
- Rome-Naples -Rome: (1929 )
- Six Days New York: (1922, 1930)
- Six Days Chicago: (1929 )