On iPhone, Gmail "new email" notifications keep appearing in lock screen?
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).