The Gavriiliada is a satiric description of the beginning of the New Testament, primarily making fun of virgin birth and God's ineptness. In Pushkin's narrative, Mary, the mother of Jesus, a young and attractive Jewish girl, is married to an old and impotent carpenter who has taken her as wife only to keep house. God chooses her to be the father of Jesus and sends Archangel Gabriel to announce the good news. Satan learns about God's plan and arrives first in the form of a snake to seduce and deflower Mary (and keep his influence over the soul of man?). Gabriel arrives a bit too late to save her from Satan but manages to drive him off with an illegal punch [Впился ему в то место роковое (Излишнее почти во всяком бое), В надменный член, которым бес грешил]. Then he quickly has his way with Mary who'd already seen him in a vision and was impatiently waiting for him. The next morning, God in the form of a dove flies into Mary's bedroom and has intercourse with her (still in the form of a dove), thus thinking He has conceived Jesus. Mary is left to marvel at all this sudden attention ("Вот шалости какие! Один, два, три! - как это им не лень?") and we are left uncertain just who it is who's fathered the Son of Man.
Although the story is highly blasphemous and satirical, it is not blatantly pornographic and is written in a fine high-spirited style (although tetrameter might have made it even crisper). Illegal copies of the text circulated in Russian society for a hundred years until it finally saw the dark of print at the beginning of the 20th century.
In 1826, during the investigation of Decembrist activities, chief investigator Bibikov came across multiple copies of the poem. However, this produced no consequences until 1828, when Holy Synod learned about the poem. Because the Holy Synod could prosecute blasphemy, a new inquest was opened. However, for some reason, the case left Holy Synod and was investigated by civil authorities.
In the summer of 1828, Pushkin was asked by the investigators about who gave him the text. Pushkin alleged that the poem circulated among Hussar officers and that he got a copy of a poem one of them circa 1820. He claimed he couldn't remember the person he got the poem from. This sounded like a poor excuse, so Pushkin was watched closely.
In the fall of 1828, Russian tsar Nicholas I asked Pushkin "in his name" to clarify the issue in a private confidential letter (Pushkin enjoyed special treatment by Russian monarch). However, politics of that time might suggest that both "formal request" and reply by Pushkin were initiated by third party. Whatever the truth is, some time later the case had "gone cold", and, although it was never formally closed, Pushkin was left alone.
In 1951, a hand-written copy of the letter of Pushkin to the tsar was found. Despite disputes of authenticity surrounding the discovery, most scientists agree that the letter is a genuine copy since it alludes a lot to the previous letter in a way that rules out chance beyond reasonable doubt.
Although Pushkin himself denied authorship of Gavriiliada, an overwhelming majority of scientists agree that he indeed wrote the poem. Usual argument against his authorship is the content of the poem. However, Pushkin was known as a somewhat rebellious person and is known, for example, to write epigrams on his superiors accusing them of homosexuality. The language of the poem is stunningly similar to Pushkin's even to the naked eye. Therefore, most likely, he indeed wrote the piece.