Gardner is a city in Johnson County, Kansas, United States. As of the 2020 census, the population of the city was 23,287. It is located withi…Gardner is a city in Johnson County, Kansas, United States. As of the 2020 census, the population of the city was 23,287. It is located within the Kansas City metropolitan area. Gardner was founded where the Santa Fe Trail and the Oregon/California Trails divided. The Santa Fe Trail sent travelers southwest through Santa Fe and Albuquerque toward Los Angeles and San Diego. The Oregon/California Trails bore west for a few miles before turning north toward the Kansas River valley, followed the Big Blue River into present-day Nebraska, followed the Platte River west, and ultimately split in present-day Wyoming, Idaho or Utah.