Great white if I'm correct is just Mycorhizae.
Mycorhizae attach to the roots in a symbiotic nature and actively extend the roots to feed better.
Trichoderma helps to fight medium pests and also helps convert decaying matter into nutrients.
GNATROLL, GNATOFF, gognats, etc are all Bascilious therengiensis products which stop the larvae from feeding.
Predatory nematodes, hypoaspis miles, pirate bugs are all known killers of fungus gnats.
Sand on the top of your medium still allows gnat access via your drain holes.
If they keep coming back you have to find out why and how?
Usually open windows or on you when you come in from outside.
J