Gmail notifications not working on Android 6.0.1 Marshmallow [Solved]
James Watt

I had this issue as well, the suggestions here did not work, however I found the issue for me. In my case, I had restricted the data usage. Check the settings under Connections=>Data Usage=>Data Saver. (possibly also test with Data Saver turned off) If Data saver is Turned off, check that gmail is listed as “Allow app while data saver on”