Archangel Gabriel, also know as Messenger of God, descended from Heaven with a legion of angels with an unknown message from God. He was gunned down by humans in Jerusalem, thereby starting the angel attacks on humans.
Gabriel is mentioned to have golden wings.
As Messenger of God, Gabriel delivers God's messages and rules to the angels.
At one point, he set the rule that no angel is to marry a Daughter of Man, else they will have 'Nephilim monsters' as their children. The Watchers disobeyed this, however, and Gabriel sent Raffe to hunt and kill them all.
Penryn explains that Gabriel had been gunned down from the pile of rubble that was Jerusalem. The footage was caught in TV and news videos.
End of Days Edit
Uriel confesses to having killed Gabriel to eliminate competition in the election for Messenger. He tells Laylah that Gabriel hasn't actually spoken to God for eons, or perhaps never.