To be fair, you are trying to do something that is really beyond the scope of its intended parameters. Because you are tying to do something that is counterproductive, that does not increase the threat level to all growers, collectively. (it is just a concern for you, in particular) Outdoor hydroponics negates some of the most critical control elements - i.e., the environmental ones - and leaves you susceptible to things that even sub-optimal growing conditions in the proper media, will seldom expose you to.
So, the threat level is still more than appropriate. Outdoor growers live with fungus gnats 100% of the time, and usually, don't even know they are there. (which they would, if their role in the transmission of pathogenic root rot were significant) By far, the biggest threat from fungus gnats, is with seedlings and damping off. And even that can be mitigated, without actually destroying the gnat larva.
They are really more of a nuisance when they find their way indoors. But other than that, one of the most overrated pests, by far.
For what it's worth, I live in the sub-tropics, and 9 months out of the year, my weather is just like yours. You are not going to find much joy in growing tomatoes outdoors hydroponically. Suggest you try a different method. (search for my modified Hempy buckets)