Ubuntu – Display problem after switching to GNOME3 from unity in 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.

One way to switch to a Ubuntu gnome is installing ubuntu-gnome-desktop metapackage. But, that you give you two DEs aren't much pure. Unity settings sometimes causes problem in Gnome-Shell and vice versa.

To completely remove all components of Ubuntu and install Ubuntu-gnome instead, I suggest using this script. The script was developed by the person who was providing the command for these kinds of Switching till 15.04.

• Download the purebuntu.py file from this GitHub repository.

• Install the apt-rdepends package using the terminal command

  sudo apt-get update && sudo apt-get install apt-rdepends
  

• Run the purebuntu.py file using this syntax:

  python purebuntu.py --remove ubuntu-desktop --keep ubuntu-gnome-desktop
  

• This will generate an apt-get command which will remove Ubuntu related packages and install ubuntu-gnome-desktop metapackage.

This will give you a fairly clean Ubuntu gnome desktop. Take note though, You might lost already installed extra packages because of this operation.

P.S.: You can generate the commands to switch back using:

    python purebuntu.py --keep ubuntu-desktop --remove ubuntu-gnome-desktop