Gardnerian Wicca is a mystery religion in the technical sense: its material is oathbound, and it is transmitted by initiation from a person who was themselves initiated.
There are three degrees. Initiation is conferred by a High Priestess or High Priest of sufficient degree, ordinarily on a candidate of the opposite sex, and the first two rites are, as Hutton observed, clearly Masonic in outline, which is unsurprising, since Gardner assembled the ritual from ceremonial magic, Freemasonry, folk practice and the poetry of others. Doreen Valiente, initiated in 1953, rewrote a great deal of the liturgy, removing Crowley material Gardner had used and replacing it with verse of her own; the Charge of the Goddess in the form most Wiccans know is substantially hers.
The working document is the Book of Shadows, copied by hand from initiator to initiate rather than published. There is no headquarters, no governing body and no central authority. A coven is autonomous; a High Priestess who has hived off from her mother coven runs her own; and the only structure connecting them is the lineage record.
That has an obvious consequence and a less obvious one. The obvious one is that Gardnerian Wicca cannot grow quickly, and does not. The less obvious one is that the lineage record is the tradition's only mechanism of accountability. There is no body that can discipline a High Priestess, and the only sanction available is that other covens decline to recognise her line. That works slowly, and it works only if people talk to one another.