I had fungus gnats last year and tried the cinnamon without great results. I resorted to an insecticide for houseplants and that did the trick. I had to spray a few times to get all of them. Good Luck.

Val

The issue almost certainly lies in the combination of soil choice and watering habits. Keeping your soils excessively wet provides an environment that makes them dizzy with the desire to reproduce. ;-)

Al

Cinnamon is used & recommended here as an antifungal for plants, not against insects/ pest.

I'd agree w/ Al here, the best way to rid oneself of FG is to stop overwatering, or change to much faster draining mix which won't be so water retentive & therefore attractive to the FG.