Why does the Torah list Gad after Reuven and Shimon?
R. Bachye (on Pasuk 20): Because he was the B'chor of Le'ah's maidservant (Zilpah).
Oznayim la'Torah: Because they were destined to belong to the same Degel - later in the Parshah. 1
Refer to 1:5:151:3.