One of the solutions is to build gnome2 and dependencies from ports, which some web sites show. So I didn't install Gnome2 and dependencies
but Xorg and dependencies from DVD-R.
First I updated ports using portsnap.
# portsnap fetch
# portsnap extract
Second I installed Gnome2 and dependencies from ports.
# cd /usr/ports/x11/gnome2
# make install clean
It required more than 10 hours and less than 24 hours and there were a lot of error messages
during building gnome2. I used pkg_add to deal with the error messages relative to the installation of some of the packages.
# pkg_add -r THE_PACKAGE_NAME
However, I didn't use pkg_add to install pulseaudio and gdm. Please make sure that you install pulseaudio and gdm from ports through building gnome2 and dependencies.
Third the size of /usr I adopted was about 16GB but it was not enough when I built jdk16 and openoffice.org-3 after having built gnome2. Therefore, I removed all files in /usr/ports/distfiles when I built openoffice.org-3.
# cd /usr/ports/distfiles
# rm -rf *
Finally I will be happy to assist you in solving the problems related to pulseaudio and gdm on FreeBSD 7.2.