Fungus gnats are, as the name suggests, attracted by fungi.
Soil works by releasing nutrients when it decomposes, mainly by fungi.
Therefore, you have to get rid of soil as a substrate. No soil, no problems.

There are quite a few soil-less media out there.

The most common one is coco coir.
It behaves quite similar to soil, so you don't have to change anything regarding how you water etc.

You have to pre-treat it sometimes however, or else it might harm your plant. If you don't wanna do that, get hydroponic grade and not the cheap one.

My substrate of choice is LECA (expaned clay balls). The method is called "semi-hydro". The inner pot is submerged 1/3 into nutrient solution, and you have a water level indicator that tells you when to water again.

Pon is also popular. It behaves like a mix of the two above.
It's a mix of zeolith, pumice and lava rock.

The said substrates are all inert however, so they don't absorb or contain any nutrients. You have to use synthetic, full spectrum fertilizer for hydroponics.

Additional, I have a few carnivorous plants around. Pinguicula are my favourite, since they are super easy to take care of.

Nepenthes are also amazing if you have the light and RO water for them :)

Either the bacteria BTI, which Mosquito Bits contains.

Or the real fix, SF Nematodes. Nematodes will live in your soil and eat various pests, depending on the kind, and SF are good at eating fungus gnats larvae. Other Nematodes can be used for fleas and stuff too.

https://www.arbico-organics.com/product/nemattack-beneficial-nematodes-sf-steinernema-feltiae

https://www.naturesgoodguys.com/products/beneficial-nematodes-sf?variant=23323522563

I've tried a lot of methods. I haven't eliminated them but they are no longer a nuisance.

Hydrogen peroxide 1:4 in water will kill the larvae in the soil. Unfortunately, it also kills some plants like parsley, thyme, oregano.

I add some pea gravel, sand, other grit to the surface of the soil when I bring my plants indoors. That seems to help deter them.

I see the most results with sticky yellow paper traps you can get off Amazon. They're cheap, effective and one sheet will last about 4ish months. Helps keep the numbers of adults down.

Bottom watering or mosquito bits.

This stuff right here was what finally did away with them for me https://www.amazon.com/gp/aw/d/B007UTE55A

Excerpt taken from the reviews: "This is a bioinsecticide. A Larvicide (Bacillus thuringiensis) or Bt, and acts on the larvae of insects. Very concentrated. Larvae eat the bacteria and essentially die. It has a short life but works very effectively to control mosquitoes and fungus gnat larvae. You need to use this product regularly to break the life cycles of gnats and mosquitoes as they constantly lay eggs that hatch into larvae. This product kills larvae only, not eggs or adult insects."

I used Azamax in the water and it killed them all off after a few waterings

I have a cape sundew I put next to anywhere I have an issue and it pretty much always catches them all

You are probably overwatering, let things dry out a bit between waterings and fungus gnats will go away.

Diatomaceous earth