I am testing using GhostBSD in VirtualBox (EFI enabled) and I tried to replace slim with gdm, with repo changed to FreeBSD 12 current,referring to FreeBSD's hand book I installed gdm with pkg and add gdm_enable="YES" in /etc/rc.conf. There was no slim_enable="YES" orignally. However GhostBSD still boot straight to slim. And I tried with sddm and lightdm , GhostBSD boot straight to slim as well. Then after I removed slim, GhostBSD cannot boot into any dm configured in /etc/rc.conf, and a weird extra message showed after boot every time. The gdm can be started manually after text login, but the light dm not. The error message still remains after I re-install slim, and GhostBSD cannot boot to the new slim with the orginal /etc/rc.conf file (without slim_enable="YES")
The message is like the following:
* Starting xconfig ...

FreeBSD/amd64 (vbox.ghostbsd-pc.home) (ttyv0)

login: can't evaluate \_SB_.PCI0.GEX0.VGA_._DCS - AE_NOT_FOUND
(I always press Enter here then)
login:
(if Enter is not pressed, user name can also be directly typed then enter password and login)

GhostBSD uses OpenRC instead of the FreeBSD RC and to remove slim you have to use rc-update. To add GDM: