I've used lady bugs quite a bit for various bug control, as for the gnats, I can see them go crazy when I water, because the gnats come out from the soil, so I know they are eating them, but their population, FG's, continue to grow. You would need a lot, and I suppose that's what you intend.. Also they cause weird problem like clogging up filters, becoming stuck in flowers,... When released, they head to the nearest water source with a die-hard swiftness.Tip- do not have a humidifier on when releasing them. They will fly straight to it by the hundreds and the one time I did this, they actually broke the machine until I took it part and cleaned it.. Hundreds maybe thousands of dead ladybugs first day.