It's a multi-step program. First, you might want to take a good look at your potting medium. Perhaps it doesn't drain rapidly enough or is simply too fine-textured. Second, make sure that you aren't watering too much (frequently), which can be easy to do it the potting medium is not a good one. Keep the greenhouse floor as clean as possible. Provide good air circulation.

Then, applications of Gnatrol (or other Bt-i) during your normal watering regime will get rid of the larvae, if you follow the directions. Repeat applications will be required, of course, until all of the existing adults are dead and gone.

I'll vouch for the BTi (Bacillus thuringiensis israelensis). I just learned about it about 2 months ago and started using it on my Hosta seedlings I have under lights. At the time then gnat population was getting to be annoying, but as of today it is almost a non-issue. I still see an occasional gnat as I water so it may not be 100% eradication, but nearly so.

BTi is the same strain of BT that is used to control mosquitoes. I just keep a small chunk of "mosquito donut" in my water tank. Replace it about every other time I fill the tank (amounts to about every 7-10 days).

Easy and safe!!