Does Drying out the Soil Kill Soil Gnats or Just the Plant?

When I brought home my new Greek Oregano from the local farmer's market, I noticed it was infested with soil gnats.  I read that the soil gnats could only live in soil that was moist and a good solution would be to let the top few inches of soil dry out completely.  The gnats should die off without wet soil.
  I followed this advice and stopped watering my greek oregano.  The soil felt dry to the touch and the plant was doing fine.  The only problem was that the soil gnats didn't die.  Today I noticed that my Greek Oregano had wilted leaves.  If you look at the photo below you can see how the leaves on the lower left branch are beginning to wilt.  This is a sign that I need to water it.

I watered today in order to save my Greek Oregano plant.  It appears that it takes more than drying out the plants to eliminate soil gnats.  If anyone has any other ideas, I'd love to hear your solutions.