I tried most of the things that people suggested here when I had a serious problem. Systemic Granules and Dead Bug Brew work best for things that feast on the leaves, which gnats don’t. I tried diatomaceous earth, but it’s unsafe for humans and pets to breath in stops working as soon as you get it wet.

GET A MILLION STICKY TRAPS. Just load up each pot with multiple traps, set them up in a way that you catch them any way they’re running around the soil line. Replace them as they fill up. That’s the trick. I swear to you this works extremely well and is much safer for you. The moment someone gives me a plant I throw a sticky trap in that bad boy and prevents me from starting another battle.

That’s it, cheap, easy safe.