There seems to be some confusion regarding the use of the word "Pagan"
Paganism is a very broad group. It includes many ancient religions, some reconstructed ones, and some evtirely new ones.
Paganism can be broken into two major groups. Ancient Pagan and NeoPagan. Ancient Pagans are those religions which are extant through an unbroken line from prehistory. These religions are generally aboriginal in nature and include Siberian Shamanism, Australian Aboriginal and Namerican Native spirituality.
NeoPaganism includes both reconstructed and new religions. Reconstructed religions are those which seek to recreate religions lost due to conquest or societal change and include Wicca, Asataru, Druidism, and Stregheria. New religions include Spiritism and New Age.
To describe Gerald Gardner as a "Pagan" is imprecise and leads to confusion. Gardner was a Wiccan, which is Neopagan in nature. He attempted to reconstruct the dead religions of Northern Europe, with argueable results. Wicca is not older than Christianity but some forms of Paganism, ie Siberian Shamanism, are as old as the human species.
Satanism is a sect or offshoot of Christianity and should not be included here. Satanism has little or nothing in common with Paganism and most Satanists will be the first to point that out.
I hope that clears up some of the confusion.