Grey Literature is any material that can be difficult to track down by conventional research methods, but are generally included as part of your review process.
"In general, grey literature publications are non-conventional, fugitive, and sometimes ephemeral publications. They may include, but are not limited to the following types of materials: reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports... etc.), theses, conference proceedings, technical specifications and standards ... and official documents not published commercially (primarily government reports and documents) (Alberani, 1990)." [The Grey Literature Report, The New York Academy of Medicine (NYAM) http://www.greylit.org/about]
Not every online database or catalog is indexed in Google. So while Google & Google Scholar are helpful tools to locate materials in this category, its important to identify specific organizations or government agencies that gather data and publish results in the area you are researching and then search those organizations catalogs etc.. Need help identifying organizations and agencies that produce data in your area of research? Contact a librarian to help you strategize.