Hi and welcome to GhostBSD.
The Nvidia drivers should just work.
After installing drivers did you run the command

nvidia-xconfig

?


Do you have an xorg.conf file?

Try
pkg delete nvidia-driver

pkg ins nvidia-driver-340

Then edit rc.conf to load the driver at boot time
Put in the kld_list section

nvidia

Then run

nvidia-xconfig

Reboot.

That should work

I forgot to mention you need to reboot.