How to install the GNOME 3 (gnome-shell) desktop environment in Ubuntu MATE 18.04?
To install gnome3 to Ubuntu, install these packages
sudo apt install gnome-session gdm3 ubuntu-desktop
During installation, the installation tools will let you choose the default display manager. You may choose
Then reboot you Ubuntu Linux and
gdm3 will be shown. Click the gear icon to select “GNOME” or “Ubuntu” as the desktop environment and you will boot to gnome-shell. You may continue installing more software as you need them.