E: Package 'gksu' has no installation candidate
Go with below commands in terminal:
wget http://mirrors.kernel.org/ubuntu/pool/main/libg/libgtop2/libgtop-2.0-10_2.32.0-1_amd64.deb
sudo apt install ./libgtop-2.0-10_2.32.0-1_amd64.deb
wget https://mirrors.edge.kernel.org/ubuntu/pool/universe/libg/libgksu/libgksu2-0_2.0.13~pre1-6ubuntu8_amd64.deb
sudo apt install ./libgksu2-0_2.0.13~pre1-6ubuntu8_amd64.deb
wget http://mirrors.kernel.org/ubuntu/pool/universe/g/gksu/gksu_2.0.2-9ubuntu1_amd64.deb
sudo apt install ./gksu_2.0.2-9ubuntu1_amd64.deb