Answer: The grit or sand layer need only be a thin one and it is free-draining enough not to bother the roots; it works more like a barrier mulch than a layer of soil. It will not stop the existing insects, but its dryness will stop the next generation because the adults will not breed in it. Luckily, fungus gnats don't live very long!
Note that if you have fungus gnats in a number of plants, it may be that you are keeping them overly wet. Most plants do best when the top layer of soil is allowed to dry off between waterings.
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