Most of the time...the gnat eggs was in the soil you bought. Happens to me all the time...
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I have read that, covering the top of the soil with sand or a paper plate/foil or something will stop more eggs from hatching. (not advice, just something I read)
Me?..I just kill them...(amono~amono)...hand to hand combat...
..I don't think they hurt the plants; but not sure.