Nine years ago I posted an entry with a FreeBSD GDM theme that I put together. I ended it by saying that I might update it to SLiM login manager if I get the notion. I finally got the notion when I was putting OpenBSD 5.6 on my trusty old Dell X1. I created an OpenBSD version and now I added a FreeBSD logo to it for a work laptop running FreeBSD 10.1. Untar in /usr/local/shared/slim/themes/ and change your theme in slim.conf to openbsd or freebsd.
I know it’s not exactly the same as the old GDM theme but it’s close enough for what I want to do with it. Hopefully you like it too.