Yesterday I was testing on a Maxx, Turbo, and Turbo 2. One of them did not show any incoming Gmail when I turned it on. I don't recall which.
I just sent a Gmail to them, turned them all on, they all had a notification.
In your Gmail app ... go to Settings and then go to the Gmail account you use. Is Notifications selected?
If that wasn't it ... perhaps you have a third-oarty app that is intercepting them? Try running in Safe Mode disabling all apps you added without uninstalling them. If it stops it was caused by an app thaqt you added.
... Thom