The goal of a URL Page Gap Analysis is to use multiple sources of page URL data on your website and then
cross reference this data to look for holes where page URLs should be, or shouldn’t be.
A great example here is using Screaming Frog (a tool that crawls your website similar to what Google crawlers do) page URL data, and to cross check that with Google Search Console data to see if there are pages on your website that Google isn’t indexing.
Another example, is finding pages in your sitemap.xml file that are of low quality, that should be removed from the sitemap so Google spends more time crawling pages of high quality.