Go into iFile and navigate to /var/mobile/Library/DataAccess
In that folder you will see a file called DeviceSpecificInfo.plist, press 'Edit' in the top right-hand corner and select the DeviceSpecificInfo.plist file. Then, press the button in the lower right-hand corner and choose copy. Press it again and paste it. Hit rename so it will make a backup of that file.
You now need to edit the DeviceSpecificInfo.plist file. Click on the file and click 'Property List Viewer' Here you will need to change the serial number to the serial number of the device you previously had Gmail working as an Exchange account.
Do you have the serial number of the old device? If not, find it here:
https://supportprofile.apple.com
Nah, I don't think so. Just set it up as you would an Exchange account and it will work fine.
I hope I helped.