Since late last year, there have been reports of Nexus devices running Android Marshmallow not getting Gmail notifications until the app was manually opened. Google has been investigating the issue for a while now and has finally come up with a fix that will be rolled out to users over the next week.
A previous sync issue involving Hotmail accounts was resolved via an update to the Gmail for Android app. However, this update will be completely resolved on Google’s end. According to a Googler in support forums, “a fix has started to roll out. This fix requires no action, and will roll out globally over the next week.” The Googler goes on to clarify that the fix was not part of February’s security patch to Nexus devices and will not require a system update.
Those experiencing the issue have reported not getting email notifications for long periods of time. For some, turning off battery optimizations for Gmail in Marshmallow fixed the problem. However, this more permanent solution is highly appreciated and hopefully will work.