Try
There are several commands that are used to configure X in FreeBSD. But the one that always seems to work is 'xorgconfig'. It worked for me when everything else failed.
It will ask lot of questions regarding mouse, keyboard, monitor etc of which monitor section is probably more important. Keep your monitor's manual ready esp the horizontal and vertical sync rates for your model. At the end it will ask where to save the file. Default is /etc/X11. Select it.
Once its done try
If you get the graphical screen then you have to edit /etc/rc.conf as follows if you want GNOME.
You need to add a line