"The western gate" is the entryway into Hell; it is Luke's escape from his grief and torment following the death of his lover. When he has committed suicide, "she will call" and they will be reunited in the place "where she is." Luke is apparently so despondent as to choose eternity with his love, even if that means joining her in Hell.
The gate is marked by vines clinging to the wall, leaves colored red in the gloom of the western sky. As the dying leaves are blown from the vines, they remind Luke of his love and strike him with the same force as the words he hears in his heart - his love calling to him. Night is approaching - "dark will end the dark" to remedy the night in Luke's eyes and spirit if he will follow the direction of the poem's voice by going to the west gate and listening for his love's call.