Numbers 1:20

“And the children of Reuben, Israel’s eldest son, etc.] Were numbered first, and next to them those of Simeon and Gad, for they were numbered according to the order in which they were to be encamped; for under Reuben’s standard were Simeon and Gad, and under Judah’s Issachar and Zebulun, and under Ephraim’s Manasseh and Benjamin, and under Dan’s Asher and Naphtali; and according to their order were the tribes numbered: by their generations: or “their generations”, the birth, descent, and pedigree of them: after their families, by the house of their fathers: according to the families and houses to which they belonged; according to the number of the names by their polls, every male from twenty…”

“These are those that were numbered--In this registration the tribe of Judah appears the most numerous; and accordingly, as the pre-eminence had been assigned to it by Jacob [Gen 49:8-12], it got the precedence in all the encampments of Israel. Of the two half-tribes of Joseph, who is seen to be "a fruitful bough" [Gen 49:22], that of Ephraim was the larger, as had been predicted. The relative increase of all, as in the two just mentioned, was owing to the special blessing of God, conformably to the prophetic declaration of the dying patriarch. But the divine blessing is usually conveyed through the influence of secondary causes; and there is reason to believe that the relative populousness…”

“The enrollment, being taken principally for military purposes (compare Numbers 1:3, Numbers 1:20), would naturally be arranged by hundreds, fifties, etc. (cf. 2Ki 1:9, 2 Kings 1:11, 2 Kings 1:13). In eleven tribes the number enrolled consists of complete hundreds. The difference, in this respect, observable in the case of the tribe of Gad here Numbers 1:25, and of the tribe of Reuben at the later census Numbers 26:7, is probably to be accounted for by the pastoral, and consequently nomadic, habits of these tribes, which rendered it difficult to bring all their members together at once for a census. Judah already takes precedence of his brethren in point of numbers (compare Genesis 49:8…”

“By their generations - That is, the persons begotten of Reuben's immediate children, who are here subdivided into families, and they into houses, and they into particular persons. Numbers 1:27”

“1:20-43 In this second list of the tribes, Gad is placed alongside Reuben and Simeon, the tribes with whom Gad would camp while Israel was on the march.”

“Num 2:10, Num 2:11, Num 26:5-7; Gen 29:32, Gen 46:9, Gen 49:3, Gen 49:4; Ch1 5:1 Numbers 1:21”