Controlled vocabulary, like the Library of Congress subject headings, is used by libraries to organize books for retrieval. Basically they identify what the book is about. The following is a list of Library of Congress subject headings that will help you explore books in the Libraries' collection. I have hyperlinked them so that you can browse instantly.
Library tip: Most academic libraries employ Library of Congress subject headings for cataloguing and organizing information, so you can use these to search just about any library catalogue.
To find literature related to gender mainstreaming, start your search query with that term, then add additional keywords to your search string to focus on particular aspects.
"gender mainstreaming" AND [enter additional search terms to identify what aspect of gender mainstreaming you’re interested in]
"gender mainstreaming" AND "public policy"
"gender mainstreaming" AND theory
"gender mainstreaming" AND policy issues
Some "best bets" for public policy content are listed on this page.