Gunnison, CO. Online Offers. The historic City of Gunnison is a Home Rule Municipality that is the county seat and the most populous city of Gunnison County, Colorado, United States. As of the 2010 census, the city had a population of 5,854. It was named in honor of John W. Gunnison, a United States Army officer who surveyed for the transcontinental railroad in 1853. Gunnison is the home of Western State Colorado University. The Gunnison-Crested Butte Regional Airport serves the valley and nearby Crested Butte with both commercial airline and general aviation flights.