Web Filter provides a useful, and often overlooked, tool to help restrict access to personal Google accounts while still being able to utilize your company's Google accounts and applications.
Navigate to Apps > Web Filter > Advanced. The option is called "Restrict Google applications" and is disabled by default. The image below shows the option when it is enabled.
When this option is enabled, only domains listed in Domain are allowed to access Google applications such as Gmail. All others are blocked by Google. Multiple domains can be specified, separated by commas: untangle.com,domain.com. Google has more information on this feature here: https://support.google.com/a/answer/1668854?hl=en
The other important item to note here is the warning about SSL Inspector. for this feature to work you absolutely must be using SSL Inspector on your network. Without it, Web Filter cannot filter this traffic.