וַיְהִי הַגּוֹרָל לְמַטֵּה מְנַשֶּׁה, כִּי-הוּא בְּכוֹר יוֹסֵף--לְמָכִיר בְּכוֹר מְנַשֶּׁה אֲבִי הַגִּלְעָד, כִּי הוּא הָיָה אִישׁ מִלְחָמָה, וַיְהִי-לוֹ, הַגִּלְעָד וְהַבָּשָׁן. וַיְהִי לִבְנֵי מְנַשֶּׁה הַנּוֹתָרִים, לְמִשְׁפְּחֹתָם--לִבְנֵי אֲבִיעֶזֶר וְלִבְנֵי-חֵלֶק וְלִבְנֵי אַשְׂרִיאֵל וְלִבְנֵי-שֶׁכֶם, וְלִבְנֵי-חֵפֶר וְלִבְנֵי שְׁמִידָע; אֵלֶּה בְּנֵי מְנַשֶּׁה בֶּן-יוֹסֵף, הַזְּכָרִים--לְמִשְׁפְּחֹתָם.
And this is the portion that fell by lot to the tribe of Manasseh—for he was Joseph’s first-born. Since Machir, the first-born of Manasseh and the father of Gilead, was a valiant warrior, Gilead and Bashan were assigned to him. And now assignments were made to the remaining Manassites, by their clans: the descendants of Abiezer, Helek, Asriel, Shechem, Hepher, and Shemida. Those were the male descendants of Manasseh son of Joseph, by their clans.
Gilad was the only one on the east side. Everyone else was on the west side.
See too Bava Batra 118b.