How to install GNOME desktop from DVD on CentOS 6 using the command line?

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http://serverfault.com – I have installed CentOS 6 in Virtualbox. Although I have chosen to install General Purpose GNOME Desktop, the GNOME desktop does not start. All I get is a black and white text prompt. I would like to know how I can install GNOME Desktop from the CentOS DVD. How do I mount the DVD image? How do I install GNOME desktop from command line? If I already have the GNOME Desktop installed, why doesn't it start? How can I check from command line that I have GNOME installled? This virtual machine does not have internet connection, that's why I want to install the GNOME Desktop from DVD. (HowTos)