According to the United States Census Bureau, the city has a total area of 4.6square miles (12.0Square kilometers), all of it land. Gilmer is a town in and the county seat of Upshur County, Texas, United States. It is best known for being the birthplace of popular music singer Johnny Mathis. The population was 4,799 at the 2000 census. The town's namesake is a former United States Secretary of the Navy, Thomas Walker Gilmer. Gilmer is also the name of a county in West Virginia.