I use kde instead of gnome, and I use Debiasn 6, so this suggestion may not work.
I have a file called /etc/kde4/kdm/kdmrc. In that file, I have this
# The relative coordinates (X,Y in percent) of the center of the greeter.
# Default is "50,50"
By adjusting the percents (25:75 would move the login to 25 percent from the left, and 75 percent down) I can move the login box around. You may have something similar.
Try searching for gdmrc on your system. You may get lucky and have a similar config file in /etc.