Google Scholar can be useful for finding grey literature, but this 2015 study suggests that it shouldn't be the only source searched:
Haddaway NR, Collins AM, Coughlin D, Kirk S (2015) The Role of Google Scholar in evidence reviews and its applicability to grey literature searching. PLoS ONE 10(9). doi:10.1371/journal.pone.0138237
Note that you'll need to take a different approach to building search strategies in Google and Google Scholar than in many other databases:
For more details, see:
Bramer WM, Giustini D, Kramer BM, Anderson P (2013). The comparative recall of Google Scholar versus PubMed in identical searches for biomedical systematic reviews: a review of searches used in systematic reviews. Systematic Reviews 2(115). http://doi.org/10.1186/2046-4053-2-115
Advanced search operators give you more control over what results Google returns:
|“Search a phrase”||Forces the specific word order||"This phrase only please"|
|"Word"||Quotation marks around a word turns off synonyms & spell checking||"kitten"|
|site:||searches a particular website (ubc.ca) or domain (.ca)||site:gc.ca|
|filetype:||Searches for a particular filetype||filetype:pdf|
|intitle:||Searches only in the title||intitle:"climate change"|
For an excellent resource on searching with Google see the Google Advanced Power Searching page.
If you plan to publish your work in a particular journal, you might check how other reviews published in that journal have described their grey literature search strategies. You can also find some guidance in the resources linked below.
Note that the number of hits Google reports is usually an overestimate. Page 1 of your results may say there are 150,000 results, when in fact there are only 400.