The package you need to install on gentoo is unreal-tournament-goty. If this
package is masked add the following to you /etc/portage/package.unmask (if
this folder and file do not exist, create them)
 
games-fps/unreal-tournament-goty
games-fps/unreal-tournament-bonuspacks

Now:

emerge games-fps/unreal-tournament-goty

It will download some things, and then bring you to the following prompt (or
something simliar to this):

 * If you do not have the CDs, but have the data files
 * mounted somewhere on your filesystem, just export
 * the following variables so they point to the right place:
 *  CD_ROOT_1 CD_ROOT_2
 * Or, if you have all the files in the same place, or
 * you only have one cdrom, you can export CD_ROOT
 * and that place will be used as the same data source
 * for all the CDs.

 * Please insert cd #1 for unreal-tournament-goty now !
 * Press return to scan for the cd again
 * or hit CTRL+C to abort the emerge.

Put the first cd in.

(in another terminal as root) mount /mnt/cdrom

Press enter in the emerge terminal

after building for a while you will come to the following prompt:

 * Please insert cd #2 for unreal-tournament-goty now !
 * Press return to scan for the cd again
 * or hit CTRL+C to abort the emerge.

(in another terminal as root) umount /mnt/cdrom

Put the second disk in.

(in another terminal as root) mount /mnt/cdrom

Press enter in the emerge terminal

Let it build.

When this finishes you will have the basics of your Unreal Tournament install,
you might want to test it out now, to launch just type (as your user): ut

Now installing the umod packs is very simple (we added them to packages.unmask
earlier), simply:

emerge games-fps/unreal-tournament-bonuspacks

This will also install the umodpack installer, which is needed for gentoo to
extract the bonus packs and install them.

That's it! Type ut to launch.