On my iPhone, for the Gmail app, I sometimes get "new email" notifications on my lock screen that were from hours ago even when I cleared them (I unlocked the device after they appeared).
Even when the lock screen is clear of notifications, then one of these from Gmail appears, it will have the timestamp from a few hours ago rather than from "now".
Best Answer
This sound weird per-se. I use the Gmail app for a long time now on my iPhone 5 and never have I received back-dated notification, especially if I removed it (via the x button) from the Notification screen.
I suppose this could occur if you leave your Gmail message unread in your inbox AND you restart your iPhone or the Gmail app.
Also, go to Settings > Notifications > Show > 1 Recent Item (instead of the default 5 recent Items).