The Horn of Gabriel
Gabriel's Horn is revealed to be a type of spell, that when the sigil is activated will draw any angels with in earshot of it to it. The only known ingredients that were revealed for it were: blood, griffin feathers and the bones of a fairy (no specific realm is necessary).
Dean learns that two of the suicide victims in Calumet City, Illinois were taking music lessons at a music store called Harry's House of Horns, where an antique horn was recently stolen. He calls Castiel with the theory that it is a holy weapon, Gabriel's Horn of Truth, that is causing people to tell the truth uncontrollably, but Castiel searches the city and does not find it. Castiel's appearance and reaction to Dean's theory implies that Gabriel's Horn of Truth is a holy weapon that has disappeared from Heaven and that it's location is currently unknown.
A loud pulsating ringing has lead Castiel to an abandoned building in an industrial area, upon entering the room which the beacon is originating from, he finds it filled with the corpses of angels, and a pulsating sigil on a wall, which soon dies out. When Castiel sends a photo of the symbol to Sam and Dean, they reveal the same sigil has been at the site of multiple homicides in Baker Hill Valley, Utah.
When Gabriel reveals himself to be alive to Castiel. He tells him the angel siren that he was hearing was in fact the Horn of Gabriel, and that it's purpose is to unite angels by calling them to it, because there is safety in numbers. However Gadreel was using it to trap and kill angels, working his way up north through Utah, collecting the ingredients from hunter friendly shops.
In the end, Castiel uses the Horn to rally a small group of angels to him, accepting his position as a leader.
Gabriel and Gabriel's Horn in Lore
Christians of the Catholic traditions refer to Gabriel as Saint Gabriel the Archangel. Islam believes that Gabriel was the medium through whom God revealed the Qur'an to Muhammad, and that he sent a message to most prophets, if not all, revealing their obligations. He is called the chief of the four favored angels and the spirit of truth. In English-speaking culture, the image of Gabriel as the angel that shall blow the trumpet blast that initiates the end of time and the general resurrection at the Last Judgment, is a familiar trope, which has no source in the Hebrew Bible or the New Testament.
Gabriel's Horn (also called Torricelli's trumpet) is a geometric figure which has infinite surface area but encloses a finite volume. The name refers to the tradition identifying the archangel Gabriel with the angel who blows the horn to announce Judgment Day, associating the infinite with the divine.