I remember the tunnel quite well. I often used it as part of a route that enabled sheltered connections beteeen Penn Station or nearby and the area of the Public Library thanks to the then open passageway between the 34th Street and 42nd Street stations of the Sixth Avenue subway at the mezzanine level. The last time I saw that pasage it was in fairly good shape, but the MTA loves closing all underground spaces that are not heavily used and using them for God knows what purposes.

The Gimbels tunnel was indeed narrow, usually dirty, and rather dreary, and had some small, cheap shops like newwsstands on the south side--which perhaps means it mignt be possible to widen it without too much expense.It was often heavily used at rush hours, but little at other times, very likely because of its unsttactive feel.
Ed Kirkland