Is it set up for an actual account? It's very strange that it would use any battery resources if it wasn't used. You could try going the app manager is settings - all apps and find the G-Mail app and disable it. I don't know how that may affect something else in the system as I haven't heard of that before.
Uninstall and reinstall Gmail in your S3. It's really strange that any such thing is happening. I haven't heard that before or you can also chose to upgrade the software, as it might be a glitch.