Gmail's having some trouble being Gmail right now…
If you're having trouble getting mail to load in the Gmail app on your phone, you might not be alone. Google is reporting an issue affecting "a majority of users", and that issue is rooted in an invalid certificate.
Says Google's apps status dashboard:
We're aware of a problem with Gmail affecting a majority of users. The affected users are able to access Gmail, but are seeing error messages and/or other unexpected behavior. We will provide an update by April 4, 2015 12:00:00 PM PDT detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change.
smtp.gmail.com is displaying an invalid certificate.
In the meantime, Gmail on the web is working, for what that's worth. We were able to access some personal accounts through the app, but our Google Apps business accounts took several minutes to load. Your mileage may vary.
Source: Google Apps Status Dashboard