I think you can use Diamacious earth on the top of the soil to kill the gnat/flies and future eggs. It’s completely safe to use just don’t inhale it a ton because it does have a small bit of silica in it.
It’s used as a dietary supplement as well so it’s food safe for animals and humans. It just isn’t for insects because it dehydrates their exoskeleton. It’ll turn kinda brown as you water on the top of it so just know that’s normal. It’s also very inexpensive.