Download the driver package from here:

http://support.epson.net/linux/en/iscan.php?model=gt-x770&version=1.0.1

wget https://download2.ebz.epson.net/iscan/plugin/gt-x770/deb/x64/iscan-gt-x770-bundle-1.0.1.x64.deb.tar.gz
tar -xvzf iscan-gt-x770-bundle-1.0.1.x64.deb.tar.gz
cd iscan-gt-x770-bundle-1.0.1.x64.deb
sudo ./install.sh

The following may be required on Ubuntu based system (rather than Debian) to move the scanner drivers in the correct directory

sudo cp /usr/lib/sane/* /usr/lib/x86_64-linux-gnu/sane/
sudo cp /usr/lib/x86_64-linux-gnu/sane/* /usr/lib/sane/

Also, iscan may need to be started as root:

sudo iscan