Re: problem regarding "sudo apt-get update" and installing gnome desktop
Your diagnosis is correct: The system is not connecting to the Internet, and it can't find the package in its package list because its package list currently consists of only the software it shipped with.
Unfortunately I know nothing about how to use DHCP manually (on the command-line) to get your computer connected; you might need to search for a HOWTO or hang on and see if someone can give you some directions.
I try to treat the cause, not the symptom. I avoid the terminal in instructions, unless it's easier or necessary. My instructions will work within the Ubuntu system, instead of breaking or subverting it. Those are the three guarantees to the helpee.