Concise Bible Dictionary:

Not used strictly for the descendants of one man. Numbers 26:2929Of the sons of Manasseh: of Machir, the family of the Machirites: and Machir begat Gilead: of Gilead come the family of the Gileadites. (Numbers 26:29) refers to the descendants of Manasseh (No. 1, in the article on Gilead) and Judges 11:1,401Now Jephthah the Gileadite was a mighty man of valor, and he was the son of an harlot: and Gilead begat Jephthah. (Judges 11:1)
40That the daughters of Israel went yearly to lament the daughter of Jephthah the Gileadite four days in a year. (Judges 11:40)
and Judges 12:77And Jephthah judged Israel six years. Then died Jephthah the Gileadite, and was buried in one of the cities of Gilead. (Judges 12:7) to No. 2. We also read of Jair a Gileadite (Judg. 10:33And after him arose Jair, a Gileadite, and judged Israel twenty and two years. (Judges 10:3)); and Barzillai (2 Sam. 17:2727And it came to pass, when David was come to Mahanaim, that Shobi the son of Nahash of Rabbah of the children of Ammon, and Machir the son of Ammiel of Lo-debar, and Barzillai the Gileadite of Rogelim, (2 Samuel 17:27); 2 Sam. 19:3131And Barzillai the Gileadite came down from Rogelim, and went over Jordan with the king, to conduct him over Jordan. (2 Samuel 19:31); 1 Kings 2:77But show kindness unto the sons of Barzillai the Gileadite, and let them be of those that eat at thy table: for so they came to me when I fled because of Absalom thy brother. (1 Kings 2:7); Ezra 2:6161And of the children of the priests: the children of Habaiah, the children of Koz, the children of Barzillai; which took a wife of the daughters of Barzillai the Gileadite, and was called after their name: (Ezra 2:61); Neh. 7:6363And of the priests: the children of Habaiah, the children of Koz, the children of Barzillai, which took one of the daughters of Barzillai the Gileadite to wife, and was called after their name. (Nehemiah 7:63)). The Ephraimites accused the Gileadites and Jephthah with being fugitives from them, but they were severely punished for their arrogance (Judg. 12:4-64Then Jephthah gathered together all the men of Gilead, and fought with Ephraim: and the men of Gilead smote Ephraim, because they said, Ye Gileadites are fugitives of Ephraim among the Ephraimites, and among the Manassites. 5And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; 6Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand. (Judges 12:4‑6)). Pekah when he revolted against Pekahiah slew fifty men of the Gileadites (2 Kings 15:2525But Pekah the son of Remaliah, a captain of his, conspired against him, and smote him in Samaria, in the palace of the king's house, with Argob and Arieh, and with him fifty men of the Gileadites: and he killed him, and reigned in his room. (2 Kings 15:25)).

Jackson’s Dictionary of Scripture Proper Names:

gentilic of Gilead