Revert from Gnome 3 to Unity

Open Terminal or CTRL+ALT+T

To install Unity sudo apt-get install ubuntu-desktop unity

To Remove Gnome sudo apt-get remove gnome-shell ubuntu-gnome-desktop

and reboot the system sudo reboot

If followed above 3 commands, you should be greeted with Unity session after reboot. In case it still doesn't comes up, open terminal and execute unity --replace

PS: by chance anything from gnome is left, follow https://askubuntu.com/a/244658/86467

On Ubuntu 17.10, this answer worked relatively well for me after rebooting (@Mayura's answer did not work for me).

1. sudo apt install unity
2. Reboot
3. At login screen, click the cog wheel next to the sign in button and select Unity.