How can I uninstall GNOME?

What version of Ubuntu are you using?
You can also use a version of Ubuntu without GNOME. For example you can download Kubuntu, with KDE. Or Xubuntu, with XFCE. Or Lubuntu, with lxde. You can also download a CORE version of Ubuntu, that contains only the system without the graphics.

Please provide the information actionparsnip has requested, and also...

Are you trying to remove GNOME on an installed Ubuntu system, or are you trying to create a derivative distribution based on Ubuntu, which does not have GNOME?

What desktop environment do you want to use instead? Or if you don't want any DE, then please explain what you do want your system to be like.

I want to unisntall it first on a intalled, for me to test. And insted GNOME I want to install the X Windows System core with IceWM. Im using the original ubuntu, version 11.10.

That is not necessarily a good way to test this, as a system created by attempting to remove all of GNOME may be different from a system that never had it in the first place. You can create a system that never had GNOME or any of the other desktop environments provided in Ubuntu releases (KDE, Xfce, LXDE) by installing a command-line only system (see https://help.ubuntu.com/community/Installation/LowMemorySystems#Install_an_Ubuntu_command-line_system but don't blacklist the restricted modules as you may want them for your GUI), then installing just the necessary xorg and window manager packages that you want.

However, if you insist on attempting to remove GNOME from an installed Ubuntu system, you can attempt to do it by removing the package called ubuntu-desktop and also whatever packages listed at http://packages.ubuntu.com/oneiric/ubuntu-desktop as depends or recommends for it, that you decide you do not want.

Tank You!

the problem is solved