The land was originally named "Boyle" after Richard Boyle, Earl of Burlington, when the land was granted by the Masonian Proprietors in 1752, but ongoing hostilities during the French and Indian War prevented settlement.

When the grant lapsed, the town was rechartered by Governor Benning Wentworth on July 13, 1763, as Gilsum, in a portmanteau of the surnames of two proprietors, Samuel Gilbert and his son-in-law, Thomas Sumner.

[3] The Ashuelot River provided water power for woolen mills.

Badger Company, makers of "balms, potions and natural remedies."

[1] The highest point in Gilsum is an unnamed hill along the town's eastern boundary, where the elevation reaches 1,657 feet (505 m) above sea level.

The racial makeup of the town was 98.46% White, 0.39% Native American, 0.13% Asian, and 1.03% from two or more races.