How to add a GUI to a minimal install of CentOS?

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http://unix.stackexchange.com – I did a minimal install of CentOS 6.3 and now want to add a GUI. I tried using yum to install a GUI, but it says it can't resolve mirror.centos.org. I tried to install networking packages but I get directory not found for /etc. How can I get a GUI? (HowTos)