These resources give specific information about how to do a grey literature search for a systematic review.

Australia

Systematic Reviews: Grey Literature [Curtin University] [LIBGUIDE]
http://libguides.library.curtin.edu.au/c.php?g=202420&p=4518195

Canada

Systematic Reviews: A How-to-Guide: Grey Literature [Dalhousie University] [LIBGUIDE]
http://dal.ca.libguides.com/systematicreviews/greylit

Searching the Literature: A Guide to Comprehensive Searching in the Health Sciences: What is Grey Literature? [University of Toronto] [LIBGUIDE]
https://guides.library.utoronto.ca/c.php?g=577919&p=4123572

Sweden

On the Quality of Grey Literature and its use in Information Synthesis during Systematic Literature Reviews [thesis]
http://www.diva-portal.se/smash/get/diva2:829465/FULLTEXT01.pdf

United Kingdom

Systematic Reviews: Searching for Grey Literature and On-Going Research [University of Oxford] [LIBGUIDE] 
https://libguides.bodleian.ox.ac.uk/systematic-reviews/searchingforgreyliterature

Searching for Systematic Reviews: Grey Literature [King’s College London] [LIBGUIDE]
http://libguides.kcl.ac.uk/systematicreview/greylit

United States

Systematic Reviews – Grey Literature [Hirsh Health Sciences Library, Tufts University] [RESEARCH GUIDE]
https://researchguides.library.tufts.edu/c.php?g=249130&p=1658806