Ubuntu – apt-get commands pausing at ‘Waiting for headers’

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)

You can do a couple of things for avoiding this. Setting the DEBIAN_FRONTEND variable to noninteractive and using -y flag. For example:

export DEBIAN_FRONTEND=noninteractive
apt-get -yq install [packagename]

If you need to install it via sudo, use:

sudo DEBIAN_FRONTEND=noninteractive apt-get -yq install [packagename]