On retirement from the British Colonial Service, Gardner moved to London but then before World War II moved to Highcliffe, east of Bournemouth on the south coast of England. There he claimed to have been initiated into a traditional coven of witches, a survival from pre-Christian times, which continued to meet in the New Forest in the south of England.
He in turn initiated a series of women who acted as High Priestesses, founding further covens and starting a tradition of entry by initiation into the 'downlines' thus created. In the UK and most Commonwealth countries someone claiming membership of Gardnerian Wicca is usually understood to be claiming initiatory descent from Gardner. The North American term British Traditional Wicca is not widely used in the UK, but in effect means the same thing.
Since one of the most important aspects of the craft tradition is understood through experience, Gardnerians keep their rituals and coven practices secret from non-initiates. In this way, each initiate is given the opportunity to find for him/herself what the ritual experience means by using the basic 'language' of a shared ritual tradition, to discover the nature of the Mysteries.
The tradition has a focus on its community, placing great emphasis on ethical conduct and reverence towards all sentient beings as central to spiritual maturity. The belief that 'ye may not be a witch alone' also extends the idea that personal growth, both intellectually and spiritually, is dependent on and affects our surroundings and the people around us. For example, Gardnerian High Priestess Eleanor Bone was not only one of the most respected elders in the tradition, she was also a matron of a nursing home. Moreover, the BW coven today is well known as a coven with many members from academic or intellectual background contributing to the preservation of Wiccan knowledge. Gerald Gardner himself actively disseminated educational resources on folklore and the occult to the general public through his Museum of Witchcraft in the Isle of Man. Therefore, Gardnerian Wicca can be said to differ slightly from many other craft practices that generally concentrate solely on solitary spiritual development.
The stress on action over words may stem from the nature of the Gardnerian craft. The tradition is often characterised as orthopraxy (correct practice) rather than orthodoxy (correct thinking), with adherents placing greater emphasis on a shared body of practices as opposed to faith.
Doreen Valiente, one of Gardner's priestesses, later identified the woman who initiated Gardner as Dorothy Clutterbuck in A Witches' Bible by Janet and Stewart Farrar. This identification was based on references Valiente remembered Gardner making to a woman he called "Old Dorothy". Scholar Ronald Hutton instead argues in his Triumph of the Moon that Gardner's witchcraft tradition was largely the inspiration of members of the Rosicrucian Order Crotona Fellowship and especially a woman known by the magical name of "Dafo". Dr. Leo Ruickbie, in his Witchcraft Out of the Shadows, analysed the documentary evidence and concluded that Aleister Crowley played a crucial role in inspiring Gardner to establish a new pagan religion. Ruickbie, Hutton, and others further argue that much of what has been published of Gardnerian Wicca, as Gardner's practice came to be known by, was written by Doreen Valiente, Aleister Crowley and also contains borrowings from other identifiable sources.
The witches to whom Gardner was introduced were originally referred to by him as "the Wica" and he would often use the term "Witch Cult" to describe the religion. Other terms used, included "witchcraft" or "the Old Religion". Later publications standardised the spelling to "Wicca" and it came to be used as the term for the craft, rather than its followers. "Gardnerian" was originally a pejorative term coined by Gardner's contemporary Roy Bowers (also known as Robert Cochrane), a British cunning man.
The group into which Gardner claimed to be initiated, known as the New Forest coven, was small and utterly secret as claiming to be a witch was illegal in Britain at the time (the Witchcraft Act of 1735 made claiming to predict the future, conjure spirits, or cast spells a crime, and likewise made accusations of witchcraft a criminal offence). When the Witchcraft Laws were replaced, in 1951, by the Fraudulent Mediums Act, Gerald Gardner went public, initially somewhat cautiously, but during the late 1950s and until his death in 1964 even courted the attentions of the tabloid press, to the consternation of some of the other members of the tradition.
The split with Valiente led to the Bricket Wood coven being led by Jack Bracelin and a new High Priestess, Dayonis. This was the first of a number of disputes between individuals and groups, but the increased publicity only seems to have allowed Gardnerian Wicca to grow much more rapidly. Certain intiates such as Alex Sanders and Raymond Buckland started off their own major traditions allowing further expansion.
A partial summary of publicly known Wiccans 'downline' from Gardner is available here