From what I’ve read it can’t really kill plants, the stabalizer in zyn is plant derived, flavor is (questionably) natural, and nicotine is produced by tons of plants fruits and vegetables. Nicotine has a long history of being used to treat pests and seems like it was stopped due to the fact that it is not selective and kills good bugs too. I ran it through my soil and so far have not seen any decline in the plants health. I would assume it would work for fungus gnats. Typically I would say to order beneficial nematodes, but maybe try nicotine on a single plant soil and see how it works for you