Blocking Consumer Accounts on Google
Apps and Restrictions rules can only be applied when your proxy is set to perform HTTPS decryption.
The application is able to block access to consumer Google Accounts for Google Services as explained in this Google Help article.
To configure this functionality, first provide the list of domains to allow access for in Proxy Settings / Apps and Restrictions / Google Apps as shown on the following screenshot.
Then enforce limits of Google Apps for specific policy.
Please also be sure to check the google domains are not added to Global Exclusions or Policy Exclusions which might prevent proxy from HTTPS decrypting them.