how bad do the plants look? are they drooping alot? i'd imagine if u have many gnats flying around, then the roots have been compromised for a while, and the plant has been drooping looking sad for days. depends how large the plant is, and how bad the infestation is. try to deal with them asap, and make sure they cannot be reintroduced. do some searches on gnats and how to get rid of them. check for other solutions if u cant get neem oil. i've used it before it works well, and it doesn't harm the plants. but there are many suggestions and methods ppl use.