This stuff is ok, have used it before, better than nothing. Perlite on the top of Coco (inch or so later) does a similar job.
De-Gnat is the shit. It's just small stones of Diatomaceous Earth, scoop it off and re-use it after each grow.
I use an inch layer of De-Gnat on-top of all my pots and have solved all my gnat issues.