Gad is placed next, because he was joined with Reuben and Simeon in the same camp and quarters, Numbers 2:10,14. Zephon, called Ziphion, Genesis 46:16.
Related Commentaries of Numbers 26:15
Numbers 26:15
15 The children of Gad after their families: of Zephon,c the family of the Zephonites: of Haggi, the family of the Haggites: of Shuni, the family of the Shunites: