First head over to http://www.meier-geinitz.de/sane/gt68xx-backend/ and get a file called PS1Dfw.usb.
Drop to a shell, cd to the directory you downloaded it to and issue the following commands...
sudo cp PS1Dfw.usb /usr/share/sane/gt68xx/
sudo chown root /usr/share/sane/gt68xx/PS1Dfw.usb
sudo chmod 755 /usr/share/sane/gt68xx/PS1Dfw.usb
Now the scanner should be just fine.