וַיְהִ֤י הַגּוֹרָל֙ לְמַטֵּ֣ה מְנַשֶּׁ֔ה כִּי־ה֖וּא בְּכ֣וֹר יוֹסֵ֑ף לְמָכִיר֩ בְּכ֨וֹר מְנַשֶּׁ֜ה אֲבִ֣י הַגִּלְעָ֗ד כִּ֣י ה֤וּא הָיָה֙ אִ֣ישׁ מִלְחָמָ֔ה וַֽיְהִי־ל֖וֹ הַגִּלְעָ֥ד וְהַבָּשָֽׁן׃
And this was the lot for the tribe of Manasseh; for he was the first-born of Joseph. As for Machir the first-born of Manasseh, the father of Gilead, because he was a man of war, therefore he had Gilead and Bashan.
Rashi on Joshua
To Mochir, the firstborn of Menashe. Since he was the firstborn, he was the first to take a portion in the days of Moshe, on the other side of the Jordan, and thus Gilod and Boshon were his portion.