"The biggest deficiency in the free software community today is not in the software—it is the lack of good free documentation that we can include with the free software."
The gdb manual might have been quoting the essay "Why Free Software Needs Free Documentation" [1] written in 1996 [2] by someone (not confirmed to be Richard Stallman AFAICT) at the Free Software Foundation. Excerpt [1]:
> The biggest deficiency in free operating systems is not in the software—it is the lack of good free manuals that we can include in these systems. Many of our most important programs do not come with full manuals. Documentation is an essential part of any software package; when an important free software package does not come with a free manual, that is a major gap. We have many such gaps today.