Grey Literature is not academic/scholarly piece of writing. It includes government documents, foundation reports, white papers, working papers, issue briefs, organizational (versus academic) research, and other kinds of policy reports. It can be challenging to distinguish grey literature from academic sources, and often enough scholars are authors of this grey literature, blurring the lines all the more. An important distinction between grey literature and academic publications are that the latter are peer reviewed by scholars with expertise in the field.
Searching for Grey Literature:
Some social science databases will index this material and it may appear in government or organizational websites or through other means rather than formal publication. The creators of these documents may be teams or individuals, and the authors are not always listed.
Unlike academic articles, grey literature doesn't necessarily go through a vetting or peer review process. As such, when using this material, you will want to have an understanding of the organizational body sponsoring the work. Who funds them? What is their mission? Are they a lobbying or political group? What kinds of information do they publish? Do other organizations cite them? Who and for what reasons? Google Scholar is a great way to find out who is citing a particular publication.