Gridley Mountain is located in Salisbury, Connecticut. It is flanked to the northwest by Round Mountain, to the northeast by Bear Mountain, and to the west by Brace Mountain. The west side of the mountain drains into Riga Lake and South Pond, then into Wachocostinook Brook, Salmon Creek, the Housatonic River, and Long Island Sound. The east side drains into Brassie Brook, Ball Brook, Moore Brook, then into the Housatonic River.