Hello, Without completely understanding of how your french drains are constructed in relation to your walls, I can share information on Fungus Gnats. Their food source is organic matter and most often found where there are houseplants. Other locations include floor drains that are relatively unused and in the duct work of certain kinds of AC units in a home. they prefer high humidity and cooler temperatures such as would be provided by houseplants sitting on a cool windowsill, basement bathrooms, and cooling ducts in the AC unit. In your references to your french drains, this may also be possible. Since they feed on the tiniest of bits of organic matter, cleaning and scrubbing the french drains especially if they are exposed might be an option. Soapy water and something like a toilet bowl brush could be used. You want to avoid chemicals that could harm the lawn or shrubs a the point of sump pump discharge. Your cleaning should also include the sump pump pit too. If you can use plenty of rinse water to flush what you have broken loose into the sump and out of the home