Gray literature is high quality research produced by government, academia, or business, but NOT published and distributed by commercial publishers. Because it isn't published in academic journals, it doesn't undergo the peer review process familiar to most academic authors. But this doesn't mean it's not created and vetted by experts! Examples:
Elka Torpey and Andrew Hogan, "Working in a gig economy," Career Outlook, U.S. Bureau of Labor Statistics, May 2016.
National Bureau of Economic Research, “The value of flexible work: evidence from Uber drivers” working paper no. 23296, March 2017
Reference books are a great place to start exploring a topic. In addition to general information, they include references that can guide you to more resources -- saving you time and effort.
Here are a couple of resources that provide helpful guidelines for writing a literature review: