Jacques:
I've found the explanation of my problem (freeze of GDM using XDMCP). It was not a GDM bug, but a bad network configuration. I've just missed to write the terminals adresses in /etc/hosts (this is notfrequently told in xdmcp documentation).Result : GDM is very, very slow to respond to new XDMCP requests, and freezes when requests exist before the launch. It's OK with just oneX-Term, slow with two, and does'nt work at all with three.
Are you sure the slowness is GDM, or is it the Xserver? XDMCP may not be the fastest protocol to support multiple terminals running on the same machine. A faster approach may be to connect the displays directly to the server and make each a different display (:0, :1, :2, etc.). Brian