Install the drivers (do this in terminal):

sudo pkg update
sudo pkg install xf86-input-wacom libwacom

(If it asks for webcamd for some older models, install that too: sudo pkg install webcamd.)
Reboot (or at least restart your desktop session).
Plug the tablet in (preferably before boot if you hit any hot-plug quirks—some older Wacoms are picky about being plugged in at startup).
Test it:Run xsetwacom list devices in terminal → you should see your stylus/eraser/pad listed.
Open GIMP → Edit → Preferences → Input Devices → Configure Extended Input Devices.
Find every Wacom entry (stylus, eraser, etc.) and set Mode to Screen.
Click Save and close.

Enable pressure in GIMP:Pick any brush/tool.
In the Tool Options dock, click Dynamics → choose Pressure Opacity, Pressure Size, Pressure Hardness, etc. (or create your own).
Test it—pressure, tilt, and the side-switch should all work.

Users on FreeBSD 13/14 (and therefore GhostBSD) report this working perfectly with Intuos S,