I had a problem with gdm and also with lxde desktop session screen resolution. This is probably problem which occurs on old monitors.
The problem is, in gdm login screen the screen resolution is set to 800x600, which is not good.
So, I found a way on how to set the screen resolution for gdm from this link.
Taken from that link, here is how to do it:
Setting xrandr commands in kdm/gdm startup scripts
Both KDM and GDM have startup scripts that are executed when X is initiated. For GDM, these are in /etc/gdm/ , while for KDM this is done at /etc/kde4/kdm/Xsetup. In either case, you can paste in an xrandr command line string into one of these scripts. For GDM, try putting them right before initctl -q emit login-session-start DISPLAY_MANAGER=gdm in /etc/gdm/Init/Default