I have installed Ubuntu 13.04 with GNOME. Now i want to remove it completely and i want the classic Ubuntu 13.04.
It depends on which particular packages you installed, but with Synaptic you should be able to remove them all. Open it and select "Installed", then use the filter to search for the genome packages. Select all of them (you can confirm you have the correct set in this list), right click and select "Mark for removal", finally click "Apply".