To launch Gnome, you don't need gdm, just :
Code:
# startx /usr/local/bin/gnome-session
Maybe you should have a look on this page :
http://www.openbsdsupport.org/gnome-GDM.html
or this one, in french :
http://wiki.openbsd-france.org/docum.../desktop/gnome
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You should try another WM, like Fluxbox or XFCE.
On my Blade 2500, I use XFCE or FVWM, cause KDE fails to launch.