Problem with gdm

Hi all, I encountered a problem with gdm 2 on my FreeBSD 4.7:

If I login from text mode and launch gdm, there is no problem.

But if I specify gdm as the default display manager by editing 
/etc/ttys, gdm won't come up . The screen splashes back and forth 
between the graphical mode and text mode several times and stopped at 
the login prompt.

If I specify xdm as the display manager, there is no problem.

I looked into /var/log/XFree86.0.log and found no error message.

My FreeBSD version is 4.7 release. I did a cvsup and got the latest 
XFree86(4.3.0,1) and gdm2 (2.4.1.3). Anybody there could give me a 
hand? Any suggestion will be highly appreciated. Thanks in advance.

Deling