How to get rid of gnats outside?
Yes, putting out bowls of cider vinegar mixed with dawn will attract and kill adults, and putting a few drops of dawn and alcohol in a sprayer filled with water will also kill some adults, as well as aphids.
BUT, the problem is not the adults, but the larvae, which come in the bags of soil from home depot and in the soil of potted plants from garden stores. The adults lay their eggs in the soil, which then take 2 weeks to hatch, grow and become adult gnats. The larvae feed on the roots of the plants which can harm the plants and also kill them.
The way to kill them is with a biological larvicide called Bacillus thuringiensis israelensis (Bti), which is sold under the name gnatrol. Its only sold in large batches directly from gnatrol, but there are a few sellers on eBay and elsewhere who sell small amounts.
I have been battling gnats for years, and have tried all the home remedies, including sanding the top soil, hydrogen peroxide, dawn, drying out the soils and OTC pesticides. NOTHING works like gnatrol. You can also buy a product called 'mosquito bombs' or 'mosquito dunks' which contain the same genus of bacteria that is used in gnatrol, Bacillus thuringiensis israelensis (Bti).
In the future, when you buy bags of soil from garden stores, you can 'sterilize' the soil by spreading it out and drying it out in the sun, although this is not guaranteed to kill the gnat larvae. Otherwise try to avoid buying plants at stores where you see gnats flying around (namely Home Depot!!!), try not to over water.