Adults gnats are about 1/8" long with dusky wings, dark hairy bodies, and look a little like mosquitoes. Larvae are 1/16" long and clear bodied with a black head. Eggs are microscopic. Each adult can lay up to 200 eggs and when the moist conditions are present, they can go from egg to adult in 10-14 days.
Enviroments that they love are humid and moist. So greenhouses and any potted plant is subject to infestation. These buggers were in my old apartment because I had my dryer in the dining room. Now it's outside and I haven't seen many gnats around my potted plants.
They are attracted by any fungi that might be growing on the soil or plants. They are around when the soil is highly organic. Although they can be a nuisance, usually their prescence means you have a living soil going on in there breaking down and feeding your plants.
If their populations get too high, and the fungal food sources get low, they will start to feed on your seedlings roots and sometimes stems. This may be the cause of alot of my seedlings just falling over dead the past 2 years. I had a bunch of gnats, but not so many that you'd think they did any harm. Losing those plants led me to believe this creature was actually harming them, so I began to kill them when I found them. Hard to do though.
Strong healthy plants are not affected at all. But younger plants are affected the most. Not only do the larva eat roots, but they spread disease to your plants. They spread things like black root rot and Pythium wilt.
Controls include water management where you avoid overwatering, especially young plants. It's crucial in winter months when air circulation, light levels, and growth rates are lower. Also yellow sticky cards catch the buggers.
Another issue is your potting mix. Fresh mix attract the gnats better. Mostly because fresh mix is extremely moist with fresh organic matter. A way to fool the gnats is to add a top layer of about a half inch of sand to your pots and garden. They usually don't burrow into the soil that far. It leaves eggs open to drying out because sand doesn't stay moist long.
Bt H-14 is an organic control in the form of Gnatrol. Use it weekly for 2 or 3 weeks. Results are slow, but effective. The best plan is to use it each season to prevent infestation instead of waiting until after you see them. Some people water their seedlings with a Bt mixture. Many commercial greenhouse growers use a synthetic growth regulator. It has little effect on beneficial insects.
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