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