Thank you drulavigne. I downloaded the gigolo source tarball yesterday and learned that it uses WAF
to build and install.
I, installed /usr/ports/devel/py-waf (I know that is discouraged by the author), built and installed Gigolo. It doesn't yet work using Scrotwm.
It took years and cost millions of lives, but I installed the Gnome desktop and Gigolo works fine using Gnome.
In linux, desktop environments like Gnome run *.desktop files in /etc/xdg/autostart at login.
These files are also loaded automatically when using GDM to log into other window managers like Scrotwm, or can loaded by an command entry in ~/.xinitrc, when using another display manager.
I'm not sure how to load the necessary files in FreeBSD. GDM uses ~/.xsession so maybe adding command to run them in there would work and it might be possible to run Gigolo in Scrotwm. It would make a useful port and PBI either way.