I have this problem too, but not just with Gmail, but every email account, sms notifications, facebook notifications, every notification probably. I get the audio notification alert and a few seconds late the vibration starts and repeats every fewe seconds until I open the text based notification. Does the M8 fly? because very tempting to throw it out the window.
Ps, all sounds settings that include vibrant that I an find are off including sounds and notifications in settings.