I recently discovered that the Gmaill app on my ICS Galaxy Nexus wasn’t actually syncing properly. When I would mark an email as archived, it wouldn’t reflect in my desktop browser, or worse, if I sent an email from my Android Gmail app, it would get stuck on “sending” forever, and never actually send the message. Rebooting wouldn’t fix this either.
Basically what happened is Gmail app is messed up and the only way to fix this is to delete the app data associated with the program. This will essentially reset the app so it gets a fresh sync again. Note: You will lose any messages stuck in “sending” mode. You will also have to add back any account specific customizations like label-specific notifications and custom ringtones for the app.
Go to System Settings > Apps > select the ALL tab > locate Gmail and select it > click the Clear Data button. Reboot for good measure. Fixed.