cool,

i use kellog soil with cast worms and all the other organic goodness crap. works well. no gnatts. but i had them (gnats) previously using some other soil....when i do see dirt gnats. i have a solution.

take a glass cup or some low dish tray. and put some beer in it. set it in your pot. the next morning you will find about 100 dead gnats in the cup. they cant resist the beer works like you wouldnt believe..add more beer as needed this works good for outdoor growing too if you have slug problems from moisture
ive also heard of people putting a layer of sand over the top of the pot

hope this helps!
growDemon