In layman's terms, what is intended by that is not a formal organization of people but rather a commonplace practice of what is called 'the craft' today. Folk practice, medicine, psychology, etc. It has its good/bad just like today's New-Age clap trap. Some practices stemmed from deep-rooted superstitions so some was harmful, while other practices were known among the common people and still rings true today.
Much of Geralds work traces back to Co-Masonry and Strega. Strega goes back to the 1500s and is remarkably similar to Wicca. God and Goddess balance and cooperation.
There are Roots in the old Celtic religions, but Witchcraft itself was not a religion until relitively reciently
Well those who practiced before the creation of Wicca with Gardner usually didn't call themselves 'witches' They would call themselves 'cunning men' or healers of various types. The term which was used by them when Wicca was developing was they were 'traditional' witches. Two names you might want to look up are George Pickingill (the book to read is "The Pickingill Papers by W. E. Liddell and Mike Howard) and then the works of Robert Cochrine such as Roebuck in the Thicket.