I have OpenBSD 5.7-amd64 installed and patched with all the latest available fixes.
I would like to have a minimal Gnome desktop environment and I did the following to my user account (not root account):
sudo pkg_add -vi gnome-session nautilus gnome-terminal gnome-menus gnome-system-monitor
After installation of the above packages, I
sudo nano /etc/rc.conf.local
and modified/added the following:
xdm_flags=NO gnome_enable=YES gdm_enable=YES
I rebooted my box and logged in to my user account.
After logging in, I am still presented with OpenBSD's default Fvwm manager, Xterm, etc.
Before making this post I had consulted the following tutorials and discovered the instructions they contained to be unworkable.
"Building an OpenBSD desktop"
"Display Manager on OpenBSD 4.7"