Grey literature is defined as reports, working papers, government documents, and other types of literature produced outside of traditional academic publishing channels. You may be able to find grey literature in some databases depending on how the database indexes materials, but to comprehensively search grey literature, you need to look outside of databases. The same is true with clinical practice guidelines - you may be able to find them in databases, but a comprehensive search should go beyond traditional databases.
You may also want to search the sites listed on the Websites page of this guide for additional grey literature, guidelines, and population information.