Grey literature exists outside of and is uncontrolled by the usual academic and commercial publishers. It includes conference proceedings, occasional and working papers, policy papers and briefs, hearings, reports, and more.
The Congressional Research Service (CRS) "works exclusively for Congress, providing timely, objective, and authoritative research and analysis to committees and Members of both the House and Senate, regardless of political party affiliation. As a legislative branch agency within the Library of Congress, CRS has been a valued and respected resource on Capitol Hill for more than a century. The products in this collection were created for the sole purpose of supporting Congress in its legislative, oversight, and representational duties."
The public portal for CRS reports can be found here: https://crsreports.congress.gov/
Brown University has access to all available CRS reports via the database ProQuest Congressional.