[3] The county is named after John Gregg, a Confederate general killed in action during the American Civil War.

It was an area developed as cotton plantations dependent on slave labor of African Americans.

The proposed name was later changed to honor Texas leader and Confederate General John Gregg, and the county seat was determined by popular election.

Members of the Negro Business League set up a cooperative store in Longview to compete with White merchants and offer African-American residents more choices for purchases.

Beginning July 10, the town had a short-lived Longview Race Riot, in which one Black man was killed, and several Black-owned homes and properties were burned.

[15] According to the 2020 census, 124,239 people resided in the county, reflecting statewide population growth during the early 2000s-2020.

In 2020, its racial and ethnic makeup was 54.77% non-Hispanic white, 20.10% Black or African American, 0.34% Native American, 1.30% Asian alone, 0.04% Pacific Islander, 0.24% some other race, 3.86% multiracial, and 19.35% Hispanic or Latino of any race, reflecting nationwide demographic diversification.

[18] These school districts serve Gregg County:[19] Areas in Gladewater, Kilgore, Longview, Pine Tree, Sabine, Spring Hill, and White Oak ISDs (in other words, all of the school districts covering sections of the county) are in the Kilgore Junior College zone.

Local media outlets are: KLTV, KTRE-TV, KYTX-TV, KFXK-TV, KCEB-TV, and KETK-TV.

Gregg subsequently became a classic "Solid South" Democratic county between 1904 and 1948, but has since become solidly Republican at a Presidential level, although the presence of a sizeable black population has permitted the Democrats to gain a quarter of the countywide total at virtually every election.