The thing about letting your soil dry out is, you don’t really wanna do that haha. Not if you’re growing organically, anyway. And I’m not even sure that works. I tried it once, didn’t water for like four days and it didn’t make a difference. The Gnatrol will absolutely kill them, though, no need to worry about it. If it’s as bad as it sounds, you’ll probably have to do two more applications, but they’ll be gone after the third one.
I watered with a combination of OG Biowar Foliar (which is something I do a couple times a grow even if I don’t have any fungus gnats) and Gnatrol a few days ago, haven’t seen a gnat since.
You really should grab some yellow sticky traps, too, though, if for no other reason than to kind of “monitor” the situation. You’ll notice fewer and fewer gnats as you change the traps out weekly.