Hi Martin,
and welcome to the Dreamlinux forums.
I guess you have downloaded the Gnome module. This module is - as it says at the top of the website - only for the Dreamlinux pendrive installation, where you just have to drag and drop the module to the apps folder to make it work.
If you want Gnome in addition to your Xfce environment, you can achieve this by typing the following in a terminal of your choice
sudo apt-get install gnome
For a smaller set of apps just do
sudo apt-get install gnome-desktop-environment
Otherwise I would recommend installing from the Gnome-iso to get a Dreamlinux optimized Gnome experience

Hope this helps!