I don't know about that.
What's the idea behind this treatment?
HOW does it work?

Stops fungus gnats from reaching the soil where they live.

I would imagine that a fungus gnat would be able to penetrate that no? Just due to their size?

I've heard of packing sand around the plant.

Who knows. Got a link to some further literature?

update on my fungus knat's, did two treatments two weeks apart. used one fourth of the circle of the mosquito dunks,put in a five gallon bucket. put the whole pot into 5 gallon bucket, and let it soak up water until could see it on the top. It's been two weeks, and I have not seen any after inspecting soil, or seen any flying after plants being disturbed, so hopefully the little fuckers are gone.

Nematodes are always good for prevention as well - Just throwing that out there.

Mentions perlite as a topper in the containers, then Carole in the comments below the main article discusses marbles, marble chips, sand, etc.

http://culturesheet.org/pests:fungus_gnats

Hey sweet, thanks Hooper.

This may be a good idea...perlite would likely be superior to marbles as it covers more efficiently, i.e. far less room for gnats to dig down. If enough is applied, and it's kept relatively dry, it should work.....I'll give it a try this summer. Thanks for the idea

Be careful trying this topping method. I have read that the gnats will just enter through the holes at the bottom of the pot. I have also read that the perlite floats when you water allowing gnats to sneak down below it.

Wouldn't perlite at the bottom in order to cover up the holes stop that, maybe replace top perlite with sand/marble chips? I have seen perlite "misbehave" in water, so I agree that perlite wouldn't be best for topping...