IUf the gnat infestation is sever at the moment, keeping them in is the best thing you can do... (while they are in you can google for info on how to treat the area with other pestices but make sure your chickens are not exposed)
I recently posted on these gnats a couple articles with advice ...if it is real bad then the advice was to keep them penned up and not let them outside... there was no "magic bullet" for the problem
You might try planting some true citronella grass round the coop/run area:
http://www.gardenandhearth.com/Plant-Gu
onella.htm
"Citronella, Grow Your Own Mosquito Repellant
The true citronella plant is a perennial grass similar to Lemon Grass, to which it is closely related. It is not the little scented geranium you find in some stores labeled as a mosquito plant. Those may smell like citronella but are about as close to citronella as the lime scented geranium is to a lime tree. Citronella is a clumping grass that grows 5-6 foot tall, the coarse, grass-like leaves are gray-green and aromatic, and are borne on cane-like stems. Other names for citronella are Nardus or Nard grass.
Growing Citronella
Citronella is generally purchased as a small plant. Make sure you are getting true citronella, [Cybopogon nardus or Citronella winterianus], and not one of the other plants that are sold in catalogs that use terms like citronella scented. If you live in zone 10 and above you can probably grow citronella in the garden. It can be used as you would use other ornamental grasses in mixed borders, or placed in the herb bed. It does not spread by runners . Propagation is done by splitting large clumps into several smaller ones. It is quite tall when mature, and rather coarse looking, so place it in the back of the garden. In the north, citronella can be grown in a large tub and moved inside to a cool, but frost proof place for the winter. Citronella is undemanding in its care. It should be grown in full sun and watered when it gets dry. It does not like to be too wet, so use care when watering potted plants. A little fertilizer high in nitrogen twice a year in the south, and when you move it back outside in the north, will keep citronella growing vigorously. "
http://www.gomestic.com/Gardening/Five-
toes.34525
Five Plants That Repel Mosquitoes
(ezxcerpt)
"CITRONELLA GRASS (see above)
Catnip
Catnip is an herb that is most commonly used to stuff in toys or feed to cats for their enjoyment. However, the oil from this plant has actually been found to be more than ten times better at repelling mosquitoes than DEET. Planting this plant near your patio or deck will help repel mosquitoes.
Rosemary
This garden herb also has an oil that repels mosquitoes. While they are attractive plants that both repel mosquitoes and can add interest to your cooking, they are truly tropical plants that are not hardy in cold climates. You can, however, grow rosemary in a pot and take it inside in the winter.
Marigolds
Marigolds have a particular smell that many insects and humans find objectionable. They are a good plant for repelling mosquitoes as well as insects that can attack vegetable plants and aphids. Marigolds are annuals with bright flowers that range from lemon yellow to dark oranges and reds....."
