Centos – Gnome Desktop Environment install error

Your /etc/interfaces file looks fine, and since you managed to run apt-get update and apt-get upgrade successfully, this indicates that your network config is good.

As to why you can't install GNOME, the package is called gnome-desktop-environment rather than ubuntu-desktop. You may want to opt for gnome-core instead, which contains less bloat than the full blown desktop environment would.

If that doesn't work, please could you edit your original question to include the error text from apt-get.

Edit 1
Since you're getting package not found for everything, check out /etc/apt/sources.list and check the repositories for universe and multiverse aren't commented out (i.e have a # at the beginning of the line). If they are, remove the # and try again.

Edit 2
For the key error, try the following command sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 16126D3A3E5C1192. (Source)

From a security point of view, installing a GUI is a bad idea as it greatly expands the attack surface of the host. Certainly a GUI does take up some resources which could otherwise be used by the VM's. Gnome will take up resources whenever it is running - regardless of whether or not you are using it.

The solution I typically use is to NOT install a GUI on the server, but install the minimal requirements (xauth package), access the machine via SSH and then run virt-manager through SSH such that it exports onto my workstation. The added benefit being I can do this from anywhere (and I can enable compression on SSH for slower connections as well)