Like on your already known Ubuntu the gnome3 package only forces other packages to be installed. And these packages can be used without having Xorg installed. Example for a remote usage of a GNOME application:
ssh -X username@ip-of-gnome-machine gnome-terminal
So: None X application requires the xorg package to be installed. Note that FreeBSDs pkg is not the same as Debians apt-get - there's no "suggested" or "recommended", which installs packages that a user might expects to be there (even if they aren't necessary). FreeBSD is not the fluffy airily hotel - it's more about a tailor-made suit, and you're the tailor.
So no, there's not a missing dependency, there's missing knowledge. And wrong expectations.