In theory, yes, but personally I don't like to add oily substances to the soil. Some people have perfectly decent results though so I'd google your subject for dosages.
If you have fungus gnats, the best way to deal with them is to start by finding where they live. Look under pots and in drip trays for standing water or soggy soil. Get rid of it- even if you have to put your plants up on racks, any standing water means breeding grounds for pests and disease.
Next, look into a pyrethrin bomb to kill exposed insects in your grow room. 2 bombs set off 3-5 days apart, plus removal of all your yucky wet areas, should completely solve your problem.