Grace,

Thanks for your question! Fungus gnats can sometimes come from potting soil but they also seem to show up out of nowhere sometimes too! Yes, a thing layer of diatomaceous earth over the top of the soil can make it harder for fungus gnats to lay their eggs, and help reduce their populations.

The best strategy to minimize fungus gnats long-term is to allow the soil/growing medium to dry between watering, especially the top 1-2 inches. Having the soil dry will decrease the survival of the eggs laid or the larvae and it also makes it less appealing to the existing gnats to lay eggs into. As soil ages and degrades, it can sometimes hold moisture longer as well, which also can support the gnats' development. Repotting every so often when soil has degraded to that point can help with the water management issue as well. If I have a plant that seems to be taking a really long time to dry out (and creating a gnat problem), I will sometimes repot it with fresh soil to help deal with the gnats as well. Bottom watering your plants can also help because the top of the soil usually gets less moist and attractive to the gnats that way.

The sticky traps you have are probably a good remedy for the adults, and should help reduce the adult egg-laying population and therefore reduce the population over time. In addition to the diatomaceous earth, you could also do a thin layer of sand or fine gravel - all will make it harder for the adults to reach the soil to lay eggs. 

A natural bacterium, Bacillus Thuringeiensis var. isralensis (BTI) found in mosquito dunks, can also work against fungus gnat larvae. Minnesota Extension suggests adding a piece of a dunk to your watering can be used to add it to your plants and address gnats. There are also some consumer-grade pesticide products that address fungus gnats, such as the systemic imidacloprid. You can find imidacloprid in a number of different types of formulations like granules, slow release spikes, or in sprays. With any type of spray though, it may require repeat applications. Always follow the label instructions on any pesticide product and ensure you are following proper safety precautions.

Here are some information sheets on fungus gnats in houseplants that may also be helpful: https://extension.umn.edu/yard-and-garden-news/how-treat-pesky-fungus-gnats-houseplants

https://extension.colostate.edu/resource/fungus-gnats-as-houseplant-and-indoor-pests/#:~:text=The%20most%20important%20strategy%20to,where%20new%20adults%20will%20emerge.

Good luck with your houseplants!