You might have users who use Gmail as a secondary email account and they may sometimes use it for business purposes, such as when your corporate email server is down for maintenance or troubleshooting. It's good to have a backup means of communicating, but that also means company information could be at risk if those Gmail accounts are compromised.
Google just recently introduced a new feature to warn users if there is activity detected on their Gmail accounts that might be suspicious. A banner will display, letting users know the location from which their accounts were accessed when unusual activity was spotted. So what qualifies as "unusual activity?" Deviation from your past patterns, such as logging on from a different country. Maybe the user is just on vacation - but maybe someone else has discovered the user's credentials and is logging on. You can read more about this new feature here: