Created by powerful Atlantean magic in the distant past, the Praying Gargoyle is a combination of Atlantean Crystal and gargoyle stone skin. The Praying Gargoyle has many uses, but all are geared towards "protecting the protectors" in some fashion.  Much as the crystals brought stone idols to life in Atlantis, the Praying Gargoyle can animate stone gargoyles. If the Praying Gargoyle is ever destroyed, it can actually regenerate in sixty years if laid on holy ground. This is likely due to the Atlantean Crystal and the regenerative properties of gargoyle stone skin. 
In 1920, Demona found the Praying Gargoyle in the catacombs and attempted to use it to destroy the humans of Paris (and perhaps beyond) by taking control of the city's gargoyle statues. In the ensuing battle with Fiona Canmore and Team Atlantis, the Praying Gargoyle was destroyed. But Demona recovered the remains and laid them in a secret hollow in the spires of Notre Dame Cathedral. ("The Last")
Demona retrieved the regenerated Praying Gargoyle from its hiding spot on the night of September 28th, 1980. She planned to make use of it to protect her race from the effects of her magical plague as part of "Operation Clean Slate", but Goliath destroyed it, thus ruining her plan and causing her to flee. ("Hunter's Moon" Part Three)St. Damien's Cathedral to retrieve the crystal. ("Invitation Only")
If Demona places the crystal on holy ground soon after retrieving it, the Praying Gargoyle will be due to regenerate once more in 2056.
It is an interesting coincidence that the Praying Gargoyle takes sixty years to regenerate since that is the exact length of time between a gargoyle's hatching and the hatching of that gargoyle's first egg. Given the regenerative nature of the Praying Gargoyle and its stone skin components, this is perhaps more than a coincidence.