Albert Barnes Commentary 1 Chronicles 27:16-22

"Furthermore over the tribes of Israel: of the Reubenites was Eliezer the son of Zichri the ruler: of the Simeonites, Shephatiah the son of Maacah: of Levi, Hashabiah the son of Kemuel: of Aaron, Zadok: of Judah, Elihu, one of the brethren of David: of Issachar, Omri the son of Michael: of Zebulun, Ishmaiah the son of Obadiah: of Naphtali, Jeremoth the son of Azriel: of the children of Ephraim, Hoshea the son of Azaziah: of the half-tribe of Manasseh, Joel the son of Pedaiah: of the half -[tribe] of Manasseh in Gilead, Iddo the son of Zechariah: of Benjamin, Jaasiel the son of Abner: of Dan, Azarel the son of Jeroham. These were the captains of the tribes of Israel." — 1 Chronicles 27:16-22 (ASV)

Gad and Asher are omitted from this list of the tribes. Similarly, Dan and Zebulon are omitted from the genealogical survey of the tribes (1 Chronicles 4–8). We can only suppose that the lists, as they came down to the writer of Chronicles, were incomplete. The “rulers” or “princes” of the tribes appear to have been the oldest lineal descendants of the patriarchs according to the law of primogeniture.