@SoGnotSOG Fungus gnats will harm your plant. Not the flying adults but the larvae in the soil; they feed on the roots. If left unchecked, they will cause harm to your grow. Can you grow without getting rid of them? Probably. Will it impact yield? Probably.
From Colorado State University:
“Fungus gnat larvae usually are located in the top 2 to 3 inches of the growing medium, depending on moisture level. They primarily feed on fungi, algae and decaying plant matter. However, the larvae will feed on plant roots and leaves resting on the growing medium surface. Larvae develop rapidly and are fully grown in two to three weeks. They then pupate in or on the growing medium.”
I think they are a common issue for most growers. Keeping them in check will help minimize any serious damage. As @Deepsix suggested, dichotomous earth will kill adults by cutting them (be careful as DE will solidify and create a hard crust on your topsoil). Sticky yellow traps to capture flying adults (they are attracted to yellow).
Most of all, do not overwater. Fungus gnats are attracted to moist and humid soils.
If you are not in flowering, you could treat with a soil treatment to kill larvae. I used a soil drench of
HydroOrganics HOGNCO16OZ 16-Ounce Hydro Organics goGNATS Liquid Concentrate Plant Supplement. Seemed to really make a difference. A little DE and sticky traps. Amazing how many are in the sticky traps!
Good luck.