Well I will do a little experiment and see if the smell of cinnamon is enough to piss the gnats off ,I've sat 2 lids in each pot filled with cinnamon so it doesn't come in contact with the soil to kill any good fungus growing.Thats entirely possible. You need to be careful when reading what others post.
They may be talking about something that works beautifully in a synthetic environment, but may destroy an organic rhizosphere.
Most suggestions I see when people are trying to help an organic grower are bad ideas but with good intentions.
Not to say cinnamon is harmful or not, as I don't know, just be careful.
We will see if this is enough