problem between gnome & gdm

[tmaurizio at alice.it]

I  am trying to build gnome and now that i am near the end, when  
arrive to compile gnome-session,
terminal say me:

################
Error: Target org.macports.activate returned: Image error: /opt/local/ 
share/xsessions/gnome.desktop is being used by the active gdm port.   
Please deactivate this port first, or use the -f flag to force the  
activation.

if I uninstall gdm, say the contrary,
can you say me why?

regards
maurizio