I know that with openbox, I installed it and didn't want it running each time I started X. (This was in Linux.) There was, in Linux, an /etc/xdg/autostart and in there something like im-settings-start. (Or something similar). I renamed it to whateveritwascalled-orig and then it no longer started when I started X.
Gnome is a desktop environment, as opposed to a window manager, so it might be harder to find its start up scripts, but it's worth looking for that file--in FreeBSD, it would probably be in /usr/local/etc/xdg, but I'm not on a BSD machine right now.