when i try to start GNOME using an AMD RX 570 then it freezes directly at start.
But if i tell Xorg to use the scfb driver (Software Rendering) then it works.
Tested with xf86-video-amdgpu and the modesetting driver from package + using drm-kmod from ports with FreeBSD 12.2-STABLE r369250.
Its a x11-wm/mutter issue. When i try to use mutter standalone i have the same issue.