Hello,
Which DLP version are you using? If you upgrade to the 14.0.1 v (agents and endpoint servers) the DLP will be able to control the Gmail traffic (using the latest versions of IE, Chrome and Firefox).
Just make sure you activated the below options in agent monitoring:
If you are using for instance the DLP 12.5v, you can still control Gmail traffic with IE but not with Chrome unless you add manually the Chrome app to the Application monitoring (is not recommended and brings performance issues, in the latest DLP version the Chrome is already natively supported).
Best regards,
Morgado