Though he carried out a brief stint as a human, Gilbert has been a demon ever since the Fall. Before all that happened, he was an angel called Pruflas. He was good enough at being an angel that he got a place in the Thrones of God, looking after all sorts of matters. It gave him a bit of an inflated ego, but it was managable. He always had his childhood friend Ezriel to knock him back down to size anyway. One day, Ezriel came to him and dragged him to see Lucifer, God's right hand angel, preaching about how God was being unfair by favoring this Jesus person. The idea of speaking out against God initially frightened Pruflas, and he ran away from the growing rebellion.
Ezriel mocked him relentlessly for his fear, and while it was meant to be friendly, it eventually got to Pruflas. Defiant that he was not a coward, and with the wriggling doubts about God planted in his mind by Lucifer, he eventually turned up to join the Rebellion, just in time for the Fall. In the meanwhile, however, Ezriel had found a new friend, an angel called Rofocale, and Pruflas took an instant dislike to him, made only worse by Ezriel practically dismissing him for Rofocale.
The Fall itself was a great blow to Pruflas, who had once been so devout. Not only that, but it was an affront to his pride. He raged and ranted and fumed that God was unworthy of him, and agreed that it was better to reign in hell than serve in heaven. Lucifer, seeing this vehement denial of God just as he was dividing the newly made demons into categories under the Seven Sins, included Pruflas, now called Gilbert, in his department of Pride.
As a demon under Pride, he is often tasked with making sure the souls of the damned go where they're meant to. They also tend to send him on menial tasks, but don't tell him that either, he'll rant and rave about being too awesome for such things and these tasks have to have a deeper meaning... right?
Gilbert's relationship with Elizaveta (Ezriel) is still as it was before the Fall, if not slightly strained because of Roderich's (Rofocale's) presence. However, what brought them a little closer was when Gilbert, bored one day in Hell, adopted a hellspawned young demon named Dumas (pronounced "Doo-mass"). Elizaveta didn't trust Gilbert to raise the kid on his own, and started helping him out, since occasionally Gilbert would have to go to Earth to tempt some humans, or while he was one parole as a human, and who would care for the kid then? Taking Dumas along tended to end... badly.
He met Francis when he was enjoying his brief parole as a human on earth. He traveled from Germany to watch the French Revolution. They hit it off quickly, and soon became friends, though Gilbert never alerted Francis to his status as a demon.
He first encountered Arthur after one of the vampire's coven accidentally summoned him from Hell. After terrorising the coven for a little while and getting into a nasty fight with Arthur, the vampire called upon his Hunter associate Gabriel, and forcibly sent him back to Hell in the most undignified manner possible, thus earning Gilbert's eternal loathing.