Gill was first settled in 1776 and was officially incorporated in 1793. The town is named in honor of Moses Gill, a member of Massachusetts' Executive Council who became lieutenant governor in 1794 and acting governor in 1799. Gill is located on the west bank of the Connecticut River, which flows along the town's eastern and southern edges.
The river forms most of the town's boundary with Northfield, to the east, and separates the town from Erving to the southeast, and Montague to the south. The Fall River, a small tributary, forms the boundary with Greenfield to the southwest. Continuing clockwise, the town shares land boundaries with Bernardston in the northwest, and a portion of Northfield to the north.