The last command does not work because I forgot to add "adb shell" in front of it. But it is not really necessary as you are going to reboot anyway.
There is one more thing you can try, you can try to delete the gmail folder in /data/data, you should be able to do that with ES File Explorer or Root Explorer, the folder is already read/write by default, or use adb if you prefer. The folder is
/data/data/com.google.android.providers.gmail
Then reboot the device and give it another shot.