The inside margin of the pages in a bound book.
Preparing Art for Printing
Bernard Stone and Arthur Eckstein
Van Nostrand Reinhold Company
(c) 1965 Litton Educational Publishing, Inc.
I can provide numerous other references on request.
There is reason for maintaining this definition. When preparing art for a two-page spread in a case-bound or side stitched publication, gutter specifications are provided by the publication that specify how much elements that cross over must be offset left and right at the butt line to appear continuous after the pages are bound.
Depreciation of this word's specificity should be immediately deprecated.