How to set unity as default after installing ubuntu-gnome-desktop in Ubuntu 16.04?

Recently I installed gnome-desktop in my Unity Ubuntu 16.04.Now I want to set my Unity desktop environment as default.

I installed Gnome by using these commands :

sudo apt-get install ubuntu-gnome-desktop

then chose gdm3 as default.

Now, I have searched on internet how to do so, one of them say this :

sudo /usr/lib/lightdm/lightdm-set-defaults -s ubuntu

But it throws error saying :

sudo: /usr/lib/lightdm/lightdm-set-defaults: command not found

I'm using ubuntu 16.04.

Add the following line to /etc/lightdm/lightdm.conf file ( if it doesn't exist - create it. File must be owned by root )

[SeatDefaults]
user-session=unity

That will make unity default for all users, but remember what mikewhatever said: if you choose a specific session once, login screen will remember it.