I've got the Gmail iOS app installed on my iPod Touch 5. Also, I have set the app to only show notifications for "Important" emails (which are indicated with a yellow tab in the web version of Gmail). Sometimes, when I get an important email, the email will show (as expected) on my iPod Touch's lock screen as a notification. However, sometimes the iPod Touch will wake from sleep to show the lock screen, but no notification appears.
Is this something that I can fix on my end? Or is it a bug with the Gmail iOS app itself?