This article explains how GFI WebMonitor's Safe Search Policy works.
Under Internet Policies > Search Engine Policy, an administrator can activate the Safe Search option, which enables the Safe Search feature available in major search engines
The Safe Search feature generally works using color shades and adult-related terms, thus, blocking all images or searches that match the parameters already set by the search engine.
Note: Google is redirecting all HTTP requests to HTTPS. So, in version 2013, for Google Safe Search, HTTPS Inspection must be enabled for the feature to continue working correctly. In version 2015, you have the Google HTTPS Redirect; with this option enabled, the traffic will be redirected to HTTP and will work even with HTTPS Inspection disabled.