Looking at the documentation for Google's custom search API, the list of country codes for the "gl" parameter isn't available.
The gl parameter value is a two-letter country code. The gl parameter boosts search results whose country of origin matches the parameter value. See the Country Codes page for a list of valid values.
The link found here: https://developers.google.com/custom-search/json-api/v1/reference/cse/list
points to here: https://developers.google.com/custom-search/docs/xml_results#countryCodes
Which doesn't contain any country codes.
Where can I find the list of valid country code values, so I can be sure of the set of valid values?
XML API reference appendices > Country Codes
https://developers.google.com/custom-search/docs/xml_results_appendices#countryCodes