Azamax (Azatrol), these products are derived from the same plant that neem oil comes from,are OMRI approved, and can be used in late stages of your bloom cycle. However, at 2ml per liter application rate, you will go broke trying to get rid of your root gnats as a 4 oz bottle costs around $25-$30. Your better bet (and way cheaper):
1) get a beneficial out there (I use oregonism from Aurora because is has the beneficial's that you need to treat the root gnats and it's cheap)
2)beneficial's need food - you can use carboload, sweet from botanicare, etc.
3)apply the gnat sticks or yellow traps at the bottom of each of your plant sites as well.
-The beneficials and food will go directly into your reservoir. I used 8oz of oregonism for 50 gal res. You can go 5 ml per gallon for the sweet.
-If you are in bloom , I have bad news for you, you will not 100% entirely get rid of the gnats, however, you will have good flowers in the end and a proper yield if you follow this regiment all the way until the week that you flush.
e.g. I had a buddy ask to help save a crop and they had fungus root gnat infestation at week 2 of bloom. I used this exact regiment to treat them (they were on the virge of death). In 3-4 days they started to eat again. In the end, the results were 23 plants (in hydroponics ebb/flow 2 gallon buckets), under 6000 watts = 6.45 lbs. Better than 0 lbs which is what they were looking at in week 2 lol