They can do 2 things. The larvae are feeding off of your roots so you want to destroy them. How far along are your plants? The #1 thing you can do to slow them down is to stop overwatering( (you might not think you are but you are). let your soil dry out before you water next, I promise you wont regret it. Another problem with the gnats is once they can fly they will fly straight up into your light die and fall on your flowers, which are sticky right, so they make your flowers look like crap. I have seen some pretty horrible fungus gnat infestations before so you want to get on it as soon as possible. I know in the northwest they are particuraly bad this year.