On my Samsung running 7.0 it is as follows:
Settings>Notifications>Advanced Settings (at top right corner)>Scroll to Gmail>Set "On Lock Screen" to "Show Content"
It's important you're notified when you receive an email when your device is locked. I will be happy to help get this corrected. Have you tried to remove your gmail account from the settings and readd it? Do you have any other email accounts that give you a notification when your screen is locked?
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