I'm getting the same issue with GhostBSD-26.2-R15.1a2-05-05-16.iso, the MATE OEM version, and the XFCE version. Right as the GUI launches the mouse cursor appears in the center of the screen and a white rectangle in the top left corner, possibly an XTERM prompt indicator. All black otherwise and unresponsive.
On the terminal I can see Xlibre start up then startxfce4 complains that an X server is already running on display 0, and then xfwm4 warning that another compositing manager is running on screen 0. The only errors in Xorg.0.log are related to failing to load module "intel" and "scfb". I'm thinking the intel module might be needed to run the integrated graphics?
This is on a 2012 MacBook Pro which I have previously been able to install the Gershwin build on