Hey guys so I got my Nexus 6p 1.5 weeks ago and as for gmail notifications they are hit or miss. I opened a ticket with a project fi team and they are aware of the issue and said it's a sync issue between GMAIL and the project if network.
Anyhow my work around for now is as follow.... I downloaded the Outlook app and the new outlook app allows you to setup a google account which supports push notifications. So far it's working great and the view is very close to the gmail app as far as email conversation threads and reply/composing a new email. It is using the new Material Design UI's so its very similar. Anyhow this is working for me and I haven't missed any notifications nor received any delayed message so far for the passed 4 days. I do hope Google fix the issue asap.
I'm still using Google Calendar for appointments and I have the contacts in the outlook app disabled which is disabled by default anyway. For me it's working ok.