I have a Samaung Galaxy Tab 7.0 plus and my Gmail app is showing I have one unread email in my inbox when all my messages have been read but I still have a "1". Any suggestions? This has never happened before.
It's possible your G-Mail app lost track of what was received and what wasn't. I think your best bet is to go to G-Mail on the WEB and see if you detect a message that you didn't see yet or any other anomaly.