The Gorgon was a lone member of a parasitic alien race from the Sarah Jane Adventures and was the primary antagonist of the "Eye of the Gorgon" story arc - inhabiting the host body of the Abbess and using the nuns of St. Agnes Abbey to try and fulfill its goal of obtaining a talisman that would allow it to open a portal to Gorgos, the home-world of the Gorgons and allow the aliens to invade Earth and presumably wipe out the human race (after enslaving them and using their bodies as vessels).
centuries ago three members of the parasitic Gorgon race arrived on Earth and inspired the legend of Medusa and her sisters, one of the Gorgons was slain by a legendary hero - the other would be slain later and thus only one "sister" remained, switching hosts until eventually finding itself the body of an Abbess by which to inhabit for nearly 200 years.
During this time the Gorgon took over the Order of St. Agnes and made them its loyal slaves, working with them on creating a large portal to Gorgos but lacking an alien device disguised as a talisman by which to open the portal.
Thus the Gorgon was forced into waiting and during this time killed many people, adding their bodies to a macabre "garden" in the Abbey consisting of their petrified remains.
By the time Sarah Jane and her family came to the Gorgon's attention its current host body was rapidly decaying and near-death - thus it became more violent than ever in its quest to open the portal to Gorgos and ordered its slaves to go forth and capture the talisman.
The Order did just that, abducting Sarah Jane and her family while giving the Gorgon the talisman, the Gorgon then prepared to kill Sarah Jane personally but was defeated when a mirror was utilized against it, reflecting the Gorgon's energy back on the Abbess and turning the body to stone : killing the Gorgon in the process.
The Gorgon, like all beings in its species, is a collective mass of ethereal serpents that must inhabit a humanoid body to survive - once inhabiting a body they enhance it considerably, allowing some hosts to live well into the hundreds but not preventing the inevitable decay of said host, thus they must frequently switch bodies as Gorgons live for thousands of years.
Gorgons have mind-control abilities and can petrify victims into stone but must leave their host temporarily to do so, tending to do so via the eyes and mouth - this is also the Gorgons major weak point as reflective surfaces such as mirrors can deflect the Gorgons energy back at their host and effectively kill them via petrification, without said host the Gorgons also perish.
- These are not the first Gorgons in Doctor Who history as the First Doctor fought the mythical Gorgon at some point, another Doctor also fought an imaginary Gorgon made real in the Land of Fiction.