Your phone wouldn't happen to be a POS HTC EVO 3D would it? About an hour ago mine started doing pretty much the same thing.
Show's my inbox and keeps scrolling to the left over and over and over. Hitting the back button does nothing, Hitting the Home button takes you back to home but a few seconds later it reopens Gmail on it's own to continue the scroll until hitting home a second time.
I've tried clearing the cache, uninstalling the updates, reinstalling the updates. Rebooting a couple dozen times (Both warm reboots and cold reboots w/ the battery pulled)
To me the behavior seems like that of a virus, so I went ahead and scanned it with the following
( Updated, Full Scan /w SD Card if available )
Lookout Premium - Negative
AVG Mobilation - Negative
avast! - Negative
Dr.Web Antivirus - Negative
Completely removed the SD Card, and tried rebooting again, No luck so I came to the Android Central forums, forgot my password for here, did the reset, after actually saying DOH! outloud, downloaded the K-9 email app, used that to reset my password for here. And after all that, here I am in the same boat.
About the only advice I can offer is delete the shortcut to the Gmail App to keep from accidently opening it. Download
K-9 Mail (Free) or some other email app from the Android Market/Goggle Play and use that until there is a fix.