What is "Gevul Lifnei ha'Ta'os Amah Achas"?
Rashi: The [inner wall of the two innermost] cells on each side were distanced an Adam from the interior of the Ulam one Amah on each side. It already explained that the Ulam (its walls) were withdrawn one and a half Amos, one to the north [of the gate's opening] and one to the south. Now it teaches that the cells outside [the gate] were withdrawn from the Ulam ? one was one Amah to the north, and one was one Amah to the south. It turns out that each cell was two and a half Amos from the interior of the width of the opening. "Gevul" is the border ? an empty place. It turns out that the thickness of the south wall of the north cell, and the thickness of the north wall of the Ulam finish together, for the north wall of the Ulam was six Amos thick, and the wall of the cell was five Amos thick; it began one Amah past where the Ulam wall began.
Radak: In front of the three cells was an Amah of width. The border of the cells was until Saf ha'Amah. The cells were open to the west opposite the Cheil, and also to the north and south. Rashi said that they were next to the eastern wall of Ezras Nashim, on the outside towards Har ha'Bayis. Since it already said "veha'Ta Kaneh Echad" (7), why did it need to teach here that each was six Amos? Perhaps it teaches that the [innermost] southern cell was withdrawn six Amos from the interior of the Ulam, and similarly for the northern cell.
Vilna Gaon: The eastern wall of the easternmost cells was one Amah. It was not really needed ? the eastern wall of the Chatzer served as the eastern wall of these cells! Rather, it was so they will have their own border (wall).
Since the cells were withdrawn from the interior of the Ulam one Amah, and that was one and a half Amos from the gate, the wall of the cell was two and a half Amos from the gate. The Ulam was inside, and the cells were outside! The wall of the Chatzer divides between them!
Vilna Gaon: Refer to 40:10:2:2.
Malbim: The cells were in the west, by Kir (the wall of) he'Chatzer. Kir he'Chatzer was their western wall. At the side of the gate they were near the gate. Our verse teaches that there was an Amah in between. In the west, the width of Kir he'Chatzer extended an Amah past the cells along their height and length. This was the border between the cells and Kir he'Chatzer. On the side of the gate, the cell wall was an extra Amah. The cell walls were five Amos thick. On the side of the gate, the width was six Amos, until the height of the opening. Two Eilim stood in the extra Amah, one on each side. They bordered between the cell and the gate. This is "v'Amah Achas Gevul mi'Po" ? i.e. on the other side open to the outside.
What do we learn from "veha'Ta Shesh Amos mi'Po v'Shesh Amos mi'Po"?
Vilna Gaon: The interior of the cell [from east to west] was six, and the thickness of the eastern wall of the Chatzer was six. Together with the Amah Gevul in between, these comprise the 13 Amos of the gate.
Malbim: The outer wall of the outer cell was six Amos. Also the cell by the gate, its wall was six Amos. This cannot teach about the interior of the cell ? verse 6 already taught this! However, the two walls separating between the cells were only five Amos ? "u'Vein ha'Ta'im Chamesh Amos" (7).
