Also you can give a good dose of Datomacious earth, especially on top of the soil. I (put on a dust mask) and sprinkle it in front of a small fan turned on high, try to coat the underside of leaves to. Works good on aphids as well. Don't use that or the neem if the plants are flowering though.
You can put out some yellow sticky traps afterward as well, available in many garden stores, or off Amazon. That does look more like some type of fruit fly rather than a fungus gnat, but all the options listed in replies hear, should help.