Gainesville is a city in and the county seat of Cooke County, Texas, United States. Its population was 17,394 at the 2020 census. It is part of the …Gainesville is a city in and the county seat of Cooke County, Texas, United States. Its population was 17,394 at the 2020 census. It is part of the Texoma region and is an important Agri-business center.