Why are the tribes listed here in order of birth, but by the Degalim (in Pasuk 24) and by the census (in 2:14), Gad is mentioned after Shimon?
Riva: Reuven is listed first, since he was the B'chor, and Shimon after him. Gad was the B'chor of Zilpah, Leah's maidservant, and he inherited together with Reuven (in Eiver ha'Yarden). However, Menasheh was not with them, since only half the tribe was in Eiver ha'Yarden, It is therefore preferable for Menasheh to be listed together with the B'nei Rachel in one Degel (R. Tam of Orleans).
Moshav Zekenim (citing R. Avraham): In the Degalim, Gad is placed among the B'nei Leah to equate the sons of the maidservants with the sons of the Imahos. 1
Oznayim la'Torah: Here the Torah lists the princes in order of birth - according to their mothers, considering Bilhah and Zilpah as one, whereas later, the Torah lists the tribes according to the four camps, placing Gad, the B'chor of Zilpah, Lea'h's maidservant, together with Machaneh Yehudah (the first of the two camps of Le'ah's sons).
