It depends how old your LT is. Like if it is a P111 with less than 256 megs of
RAM. You would need to stick with Xubie. So if you give some specs on the LT
like what is the model of the Toshiba LT. You may get a better response on it.
There is a way to install Gnome too and you can select the option on when you
log in at the splash screen on what type of you want to use. So if you have not
really custom your Xubie and do not mind reinstalling if it fails.
Open up the Terminal and lets start. We need to update the system and input the
following

sudo aptitude update

After the update now we will type in

sudo apt-get install ubuntu-desktop

It is going to take a bit on the system to DL and bla bla bla. When all said
and done. Reboot the system and when you going to log in. Go to the options in
the splash screen and select Gnome. Then proceed to login and it will boot up
Gnome. If that does not work. Reboot and select Xubuntu as login. Open up the
Terminal and remove Gnome by inputting this.

sudo apt-get remove ubuntu-desktop
and then
sudo apt-get autoremove

All this may break it and you'll may need to do a fresh install on Xubie. It is
up to you. However it may be a way to lie to get Ubie going on your LT. Remember
if you have low ram speed. Like 128 megs of ram. Gnome is going to hang up or
move really slow. This you may need to bump up the RAM. You can by new or used.
OK I babbled.