The Gateway Guardian (originally from The Princess and the Frog) is a voodoo spirit that guards the portal between the mortal world and Underworld. In that same movie, it dragged Dr. Facilier down to Hell. The portal was stationed inside Father's Mansion and he and his demonic friends used it to go between worlds, sometimes.
In Operation: ANCESTOR, when the main group infiltrated Father's Mansion to find the portal, Facilier had to face the spirit and gain them entry. He defeated it, allowing them to travel back and forth between the two worlds at will.