Land of Gad
The Philistine army was like sand on the seashore at Michmash, so the men of Israel hid, while others crossed the fords of the Jordan to the land of Gad and Gilead. Saul remained at Gilgal. (1 Samuel 13:5-7)
Prophet Gad
David remained in the stronghold at the cave of Adullam with his company of 400 men until the prophet Gad said to him, "Do not remain in the stronghold; depart, and go into the land of Judah." (1 Samuel 22:1-5)
The prophet Gad was David's seer. (2 Samuel 24:12)
The plague in Israel and the altar
The LORD came to the prophet Gad, David's seer, and said, “Go and say to David, ‘Thus says the Lord, Three things I offer you. Choose one of them, that I may do it to you.’” Gad told David, “Shall three years of famine come to you in your land? Or will you flee three months before your foes while they pursue you? Or shall there be three days' pestilence in your land? Now consider, and decide what answer I shall return to him who sent me.” Then David said to Gad, “I am in great distress. Let us fall into the hand of the Lord, for his mercy is great; but let me not fall into the hand of man.”
The plague came for three days. When David asked the LORD to let His hand be against him and his father's house, Gad came that day and said, "Go up, raise an altar to the Lord on the threshing floor of Araunah the Jebusite.” David obeyed and offered burnt offerings and peace offerings, and the Lord responded to the plea for the land. The plague was averted from Israel.