To be able to add the colemak to the layout in the keyboard preferences, you need to edit the file /usr/share/X11/xkb/rules/xorg.xml
In this file find the node <layoutList>
and copy this snippet of xml inside:
<layout>
<configItem>
<name>colemak</name>
<shortDescription>Colemak</shortDescription>
<description>Colemak see colemak.com</description>
</configItem>
</layout>
This will tell gnome that you have the colemak layout installed and you will be able to choose it in the list when you add a new layout to the keyboard preference tool.
If you can't find the path /usr/share/X11/xkb/rules/xorg.xml , you may want to try the following paths
/usr/lib/X11/xkb/rules/xorg.xml
/usr/lib/X11/xkb/rules/xfree86.xml
It depends on the version of xorg/xfree86 you have installed
Hope this helps anybody