This contradiction could just as well have been titled: Two variant traditions on how to legitimate the possession of Gilead.
If we compare the genealogies of Manasseh’s sons as portrayed in Num 26:29, 36:1, Josh 17:1, and 1 Chr 2:21-22 (P) with that of Num 32:39-42 (J) we notice some interesting discrepancies. Not surprisingly, these variant genealogies have their origins in two different traditions: the former list of passages come from the Priestly tradition (or his redactional work), while the latter most likely from the Yahwist tradition. Visually this is what we have:
In other words, J’s command to have Machir go take Gilead, which itself contradictions the traditions in Numbers 21 & Deuteronomy 2-3 (see #317-318), is represented in P’s tradition by making Gilead the son of Machir!
And this brings me to the second contradiction which in actuality is a bit unfair. The name “Gilead”—as with other names in the Torah’s genealogical traditions—is both a personal name and a toponym (a place-name). Usually when such a name appears in a genealogy it’s an indication that the locale of which the name is a toponym is part of that particular tribe’s possession, or geographical genealogy.
What is interesting here is that in the Yahwist tradition of Numbers 32:39-42 Gilead is only used as a place-name. In other words, the possession of Gilead by the Machirites in this tradition is indicated by the fact that all of Machir’s sons now possess a portion of the Gilead. In the Priestly tradition, however, the possession of Gilead is first and foremost indicated by the genealogical fact that Gilead is Machir’s son—a “fact” which then legitimates the possession of Gilead by the Machirite clan. The genealogy, then, serves a political purpose—to legitimate Israelite possession of Gilead by claiming—by creating an archaic story which states—that the ancestry of the Gileadites are Israelite.