Witches are a supernatural species.
The origins of the Gates witches are currently unknown, but they seem to be mostly human with supernatural knowledge and more the capacity to manipulate ingredients than actively use magic.
They seem like normal human beings. They have as of yet, exhibited no distinct differences physiologically from other humans.
Like many myths regarding witches the Gates witches have the ability to manipulate magic and have both access to some sort of "black magic" (Devon Buckley), and some sort of "white magic" (Peg Mueller). They have not yet exhibited the ability to actively "cast" magic, and seem mainly to manipulate potions and other objects to achieve results, medicines: (Jurisdiction) to heal people and raise their immune systems, (The Monster Within) teas (Pilot) that made people easier to hypnotise and control, facial scrubs (Jurisdiction) that could make people younger. It is not indicated whether magic is innate in all humans or only a select few. There are at least several uses of magic as indicated by the variety of uses it has been seen to have. However Mia blogs that she moved an object with her mind. and devon is shown teaching Mia to put out a candle with her mind suggesting some sort of elemental control.
Dark Magic has side effects, Devon brought Charlie back to life but with intended or unintended consequences as he has been transformed into an unknown Supernatural being with red eyes.
Peg indicated that Devon was using magic to "lure" her clients, and that her teas were doing something to them. It is also further hinted that Chad Taylor was under her spell when he presumably shot the old chief.
It's currently unknown if there is any other types of witches out there in The Gates.