Having installed a basic LXDE-Openbox environment, we want to boot into it now.
When back at the GDM Login Screen, click on Sessions again, and select LXDE. Enter your username and password again in order to log into LXDE.
Removing XFCE to leave a "barebones" system is somewhat difficult, so I used aysui's suggestion to remove XFCE and all the Xubuntu stuff in order to leave just Gnome behind.
So, once in LXDE, click on the Terminal icon in the Quicklaunch Panel, and enter the entire Pure Gnome Remove Xubuntu command from aysui's tutorial, just leaving out the final && sudo apt-get install ubuntu-desktop (which would obviously install Gnome).
This removes most of the XFCE desktop, but leaves Xubuntu's "borrowed" system settings GUI controls and games from Gnome.