I would suggest you install ES File Explorer (free in the market).
When you launch this app, you have "Manager" in the context menu. There you can click on Gmail, and in the next window you have the option to clear the cache.
If that does not work, you need to re-install gmail. If it is installed in the data partition, you can uninstall with nookcolor tools, ES File Explorer or any other tool that manages apps. On the other hand, if it is installed in the system partition, you have to delete it using Root Explorer, you mount the system partition as read/write, then delete the file.
Afterwards, you can download the file from the internet (it's in the market) and install, just copy it to the SD card and click on it with ES File explorer to launch the installer.
Gmail App
If the file was originally in the system partition, I would recommend moving the installed file back to the system partition, though this is not a must. Basically, having an app in the system partition means that it is protected and can't be accidentally removed (you need root permissions for that).