You don't need neem or any other oils, nematodes or anything else. I do recommend covering the top of the soil, but not with sand, they can go right thru it, and not with DE, it doesn't work when wet. I am using the fabric from old smart pots cut to fit the top of the pot. This also helps to keep the pot from drying out too quickly.
My gnats came from a bag of Ocean Forest, I am absolutely sure about that. Also, pasteurizing the soil with boiling water works, but you have to get the temperature of the soil up to at least 140-160 degrees F for 15 - 30 minutes, and be sure to stir and mix thoroughly. It is a giant pain in the ass, but it works.
I am looking for a used pressure cooker to make this process easier and fool proof in the future. I also tried cooking the soil in the oven, it takes about 15 minutes at 350 degrees F for 15 minutes, then another 45 minutes at 250 degrees to really get the soil up to 160 F in the center (this was a roasting pan filled with soil). I like the boiling water method better, it makes the soil perfectly moist for transplanting.
I am using
Xtreme Gardening HGC721205 Mykos Pure Mycorrhizal Inoculant Organic Root Enhancer, from Amazon, to help the soil:
View attachment 5094629
Be sure to cover any drain holes if you are not using fabric smart pots - the gnats love drain holes, and unless you dunk the pot into the BTi treated water you cannot be sure all the holes will be saturated. I would recommend dunking at least once, I cannot do that because I am disabled and cannot lift a pot that heavy.
Lastly, my experience was by the time I saw a gnat flying around there was a full blown infestation, I just couldn't see it. Once I started putting the yellow sticky traps out, I was catching 15-25 gnats in my traps EVERY SINGLE DAY for 8 weeks in flower (20 gnats x 56 days = 1140 gnats). However, despite catching so many every day, I never actually saw any flying around when I looked in the tent. So, I would bet that you have far more gnats than you realize, something that will become obvious when you put out the traps.