I think he means the vertical strip on the lefthand side of the tray, adjacent to the box's "spine." (On a standard jewel-case with a black tray, the gutter is covered/hidden. But with a transparent tray, the gutter is visible. Although I suppose he could mean the verso of the spines--the inside-facing portion of the "wings" of the traycard.)“Gutters” meaning interior of the Jewel Box?