From BiblePortal Wikipedia

People's Dictionary of the Bible [1]

Gad, the Tribe of. The territory given to the tribe of Gad lay east of the Jordan, north of that allotted to Reuben, and south of that given to Manasseh on that side of the river. It extended from the Jordan eastward to Aroer,  Joshua 13:24-25, including half of Mount Gilead and half of Ammon.  Deuteronomy 3:12;  Joshua 13:24-25. For physical features and history see Gilead. Its chief cities were Ramoth-gilead, Mahanaim, Heshbon, and Aroer. This tribe, in the wilderness, was placed with Simeon and Reuben on the south of the tabernacle; with Reuben and the half of Manasseh, it occupied the pasture grounds on the east of the Jordan. It was warlike, as is graphically stated.  1 Chronicles 12:8. Two famous men came from Gad—Barzillai,  2 Samuel 17:27, and Elijah,  1 Kings 17:1. The territory was the battle field for wars between Syria and Israel.  2 Kings 10:33.

Fausset's Bible Dictionary [2]

The "seer" of king David ( 1 Chronicles 29:29). "The acts of David" were recorded "in the book of Gad the seer." He joined David while in "the hold," having probably first become acquainted with David in the latter's visits to Samuel and the schools of the prophets, and by his advice David left it for the forest of Hareth ( 1 Samuel 22:5). At the numbering of the people Gad was Jehovah's monitor to David ( 2 Samuel 24:11-19;  1 Chronicles 21:9). He also took part in arranging the musical services of the temple ( 2 Chronicles 29:25). Jerome compares Gad to Elijah in the abruptness of his introduction; this concentrates all attention on his work and message, none on himself.

International Standard Bible Encyclopedia [3]

(גּד , gādh , "fortunate"): David's seer ( ḥōzeh ,  1 Chronicles 21:9;  1 Chronicles 29:29;  2 Chronicles 29:25 ), or prophet ( nābhı̄ ); compare  1 Samuel 22:5;  2 Samuel 24:11 ). He appears (1) to advise David while an outlaw fleeing before Saul to return to the land of Judah ( 1 Samuel 22:5 ); (2) to rebuke David and give him his choice of punishments when, in spite of the advice of Joab and the traditional objections (compare  Exodus 30:11 ), he had counted the children of Israel ( 2 Samuel 24:11;  1 Chronicles 21:9 ); (3) to instruct David to erect an altar on the threshing-floor of Araunah when the plague that had descended on Israel ceased ( 2 Samuel 24:18;  1 Chronicles 21:18 ); and (4) to assist in the arrangement of Levitical music with cymbals, psalteries and harps (compare  2 Chronicles 29:25 ). Of his writings none are known, though he is said to have written a history of a part of David's reign ( 1 Chronicles 29:29 ).