Canonical will stop to invest Unity and will change defalt desktop environment from Unity to GNOME in Ubuntu 18.04. This article will describe using GNOME as desktop environment in Ubuntu 16.04.
1 Install GNOME
Install gnome package. Use lightdm because gdm3 does not work correctly. You can skip gdm3 interactive setting with setting DEBIAN_FRONTEND to noninteractive.
$ sudo DEBIAN_FRONTEND="noninteractive" apt install -y gnome
2 Select desktop environment
Select desktop environment in unity-greeter.
This is used in Fedora.
2.2 GNOME Classic
This is used in CentOS.