Check the in the gmail settings and you account there are lot of sub menu to toggle on in the app..I get mine instantly while turning them on and pushNot sure if this a phone thing or an app thing. I have to go in and manually refresh for notifications, almost all the time. Very rarely do I get my notifications timely. What setting am i missing?
One of the ways 10 optimized battery is by not allowing apps like Gmail to receive email when they're not running. Eventually, as other email apps get updated to 10, they'll exhibit the same behavior. (Taking the app off optimization fixes it, at the cost of some battery.)