I can sympathize, fungus gnats have made a mess of my houseplants and lily seedlings multiple times. Plus the adults are attracted to the light of pc monitors and bathroom mirrors. My family does not appreciate that one bit, strangely enough.
I've also used Neem oil indoors as a soil drench to control fungus gnat larvae. Not sure how the cost would compare between Gnatrol and Neem per the three treatments needed to break their life cycle. In my greenhouse, imidacloprid products are very effective as a soil drench, but this is getting into a more serious insecticide that you need to wear the appropriate protective gear to use. It would work outside as well, but use only as a soil drench on non-edibles and plants not being visited by pollinators, as these products will also knock out friendly insects. Never use as an aerosol indiscriminately outdoors.
Running pots as dry as possible will lessen fungus gnat larvae. You can also put about 1/2" of sand on top of the soil of your pots. Fungus gnats won't lay their eggs through the sand.
Buy enough gnatrol to treat all your plants three times, then try sand on the soil to deter future outbreaks?