To kill gnats effectively, use a combination of natural remedies and preventive measures:

• Use vinegar traps made with apple cider vinegar and dish soap; the vinegar attracts the gnats and the soap traps and drowns them.
• Apply natural fungicides like chamomile tea and cinnamon to soil to kill larvae by making the soil inhospitable.
• Use potato slices buried just under the soil surface; gnat larvae are attracted to and get stuck on the potatoes.
• Drench soil with a hydrogen peroxide solution (1 part 9% hydrogen peroxide to 6 parts water) to kill larvae and eggs.
• Apply food-grade diatomaceous earth on the soil surface to shred larvae and gnats on contact.
• Use bacteria-based mosquito dunks or bits (containing Bacillus thuringiensis) watered into the soil to kill larvae.

Additional tips:

• Avoid overwatering plants and allow the soil to dry between waterings.
• Remove standing water and rotting fruit that attract gnats.
• Set up sticky traps or bug zappers to catch adult gnats.
• Repeat treatments over several weeks for best results.

This combined approach can eradicate gnats from houseplants and indoor areas effectively.