Gilmer is located just northwest of Longview at the junction of US 271 and Hwys 154, 155, and 300, in Upshur County, where it is county seat.
In its early years Gilmer served as a cotton-ginning center; it once had six gins in operation, and one continued until the 1950s.
In 1890 farmers in the county began producing sweet potatoes but had to quarantine the crop in the late 1920s because of an infestation of sweet-potato weevils. When the quarantine lifted, residents organized the East Texas Yamboree, a fall festival to celebrate the sweet potato harvest; the first festival was held in October 1935. In the late 1980s the annual festival continued, with some 30,000 to 40,000 people attending in 1987.
In 1990 the population was 4,822, and in 2001, it opened its thousand-acre Gilmer Lake to the public.
Gilmer is part of the Longview-Marshall, Texas metro area.