First Add Debian 9 sources. DELETE file after install… .

cat <<EOF | tee /etc/apt/sources.list.d/debian9.list deb http://archive.debian.org/debian/ stretch main
deb-src http://archive.debian.org/debian/ stretch main

deb http://archive.debian.org/debian-security stretch/updates main
deb-src http://archive.debian.org/debian-security stretch/updates main

deb http://archive.debian.org/debian/ stretch main contrib non-free
deb-src http://archive.debian.org/debian/ stretch main contrib non-free

EOF

Update package cache

apt update

Install necessary build tools

apt install -y build-essential debhelper dpkg-dev

Install build-dependencies for gksu and compile its source package

apt build-dep gksu

apt source --compile gksu

Install self-compiled gksu packages:

apt install -y ./*gksu*.deb

Remove Debian 9 repository from system for safety

rm /etc/apt/sources.list.d/debian9.list

Credits: artful