When I talked about the Gap in this particular scenario, I was looking at the truth of my library system, and library systems evrywhere - most populated areas have many more people in them than there are registered library card holders or even people that use the library on a frequent or infrequent basis. While some of these people won't actually think of the library, because there's nothing there that they want, there's a prcentage of people who really would become users of the library if some conditions that were getting in their way were removed, or barriers to their entry taken care of.
There are lots of reasons why someone might not come to a public library, whether it's local to them or somewhere else. I think they fall into some broad categories, and that recognizing those categories can help public ibraries find some of those users that they've been missing out on.