Seminole is a city in and the county seat of Gaines County, Texas, United States. Its population was 6,430 at the 2010 census. Seminole and G…Seminole is a city in and the county seat of Gaines County, Texas, United States. Its population was 6,430 at the 2010 census. Seminole and Gaines County are home to a large population of German Mennonites from Russia that came to West Texas in the 1980s.