I am a soil grower and ive had a few problems with gnats. Luckily even when you have massive amounts of them you will still see to your harvest, and the bud will probably be good. Only your harvest weight will be affected, and the plants will be more susceptible to rot, and disease.
I tryed drenching the soil with neem oil, and a drop of dishwashing soap.. it slowed them down but didnt get rid of them. I also brought my humidity down to 30% and watered from the bottom of the pots after the neem drench... and added sticky traps face down on the pots so if the gnats take to fly off they get stuck right away.
All this was until i receive a product i had ordered online called GNAT OFF,
you use 1ml/Liter of water... and after 2 or 3 applications the larvae will be gone. With soil, i realized when i lifted the pots a strong piss like smell, and a little puddle of water under them with loads of dead smelly gnat larvae... so you can actually see the little bastards dead....
The sticky traps will catch adults that way they dont lay any more eggs... (adults can lay up to 200 eggs..) so if you see 10 adults flying about imagine how many eggs.... OUCH...
but gnat off was effective, along with the sticky traps.. and the low humidity...
gnat off is basically a bacteria called bacillus thuringiensis and it is 100% organic... IT WILL STARVE THE LARVAE in the root ball, till they die and leave your pretty plants alone
PEace out
and good growing