The Gnats arent the problem it is the larva they leave in their soil. I had the same problem, Turned out after a little research that they were fungus gnats and their larva feast on your plants roots.
There are a few different ways to get rid of them.
Their larva need the soil in order to survive so one thing you can do is put a 1in. layer of sand on top of the surface. This is a not a over night fix, all it does is prevent the larva from getting to the soil after they have been layed. Eventually the larva die, the adults die and problem is solved.
You can also use 3-in-one insecticide, fungicide, miticide. This stuff will knock those bugs out the box in a day or two but since your plants are so young might do them damage as well.
My third and final suggestion is diatomaceous earth. Its organic and will kill any bug that walks over it. Tiny fossils that shreds any bug matter that comes upon it. Just dont breath this crap in, comes in a powder form.
Those fly strips only eliminate the nuisance but not the cause of it.