Hey Growmie,
There are a few things you can do you start by placing yellow sticky cards in the grow area and also get a fan blowing air over the top of the growing medium. This helps dry out the top layer of your soil and also helps prevent the fungus gnats from being able to fly around and lay more eggs which is more than likely what you're dealing with. Don’t water your plants for a few days. You want to begin drying out the soil to kill as many larvae as you can before you apply a treatment (which will involve watering). Fungus gnat larvae need a warm, wet environment near the surface of your soil to grow. Even after you’ve gone through all these steps and believe fungus gnats are gone, make sure to avoid overwatering plants, as this is the most common reason growers get fungus gnats in the first place.
Hopefully, I answered your question and remember ..."Happy Growing" and "Free your mind one puff at a time".