To complement the keyword search, you can perform a citation search of the publications to be included in the review. This involves manually going through the articles' reference lists.
Citation tracking can also be done "forwards," where you check publications that have cited the included articles. Use a database with a citation index, such as Web of Science or Scopus.
If there are other systematic reviews on related topics to yours, it is also a good idea to examine their reference lists.
Google Scholar is a search engine that retrieves scholarly articles as well as grey literature such as reports. The options for advanced search and reference export are rudimentary, but the free software Publish or Perish can facilitate bulk export of, for example, the first 100 or 200 results. It is not possible to export a complete set of results, as Google Scholar only displays up to 1000 results.