A Devil's Gate is a doorway into Hell. There are a few known Devil's Gates that exist on earth. The first Devil's Gate we see is in Calvary Cemetery, southern Wyoming. It is placed at the center of a 100-mile Devil's Trap, which, according to a demon, was built by Samuel Colt some time before 1861. Kevin Tran told Crowley of a Devil's Gate but it is unknown wether it exists or not. Billie the Reaper created a Devil's Gate using enochian sigils in "Devil in the Details".
Azazel sent Jake Talley to the Devil's Gate, and Jake used the Colt as a key to unlock and open the Gate. Sam, Dean, Bobby and Ellen were able to shut the Devil's Gate again, but not before hundreds of demons and spirits (including John Winchester, Ruby, Lilith and the Seven Deadly Sins) escaped through the Gate from Hell.
Sam mentioned that during the four month period when Dean was in Hell, he attempted to open the Devil's Gate to free Dean's soul from Hell, but was apparently unsuccessful.
In 2012, the prophet Kevin Tran tricked the current King of Hell Crowley into waiting near a supposed Devil's Gate that he was going to open, but it was a ruse and Kevin was actually performing a spell that would destroy demons.