Gnome-shell requires acceleration of to work. For this a nvidia card
with the nvidia-driver, or a Intel/ATI KMS enabled card is needed.
For the nvidia users no other changes are needed. For Intel/ATI KMS
driver the user needs to have read/write permissions on
/dev/dri/cardN (usually N=0). If gnome-shell is launched from gdm, the
gdm user also needs this access.
Access can be granted by adding the user(s) to the video group.
Additional for FreeBSD versions before FreeBSD 11.0-CURRENT revision
286524. And any FreeBSD 10.x version and FreeBSD 9.3-Release.
By using a devfs.rules(5) to change the mode of the /dev/dri/card0 on creation.