כָּל־עָרֵ֥י הַגֵּרְשֻׁנִּ֖י לְמִשְׁפְּחֹתָ֑ם שְׁלֹשׁ־עֶשְׂרֵ֥ה עִ֖יר וּמִגְרְשֵׁיהֶֽן׃ (ס)
All the cities of the Gershonites according to their families were thirteen cities with the open land about them.
Read midrashic interpretation of Joshua 21:33: and classical rabbinic exegesis.