Adrian Henry, of Finsbury Park, north London, was told by Sir Lawrence Verney, the Recorder of London: "You and they showed no mercy, and you cannot now expect that mercy will be shown to you."
The judge expressed amazement that Henry, who is 6ft 1ins tall and weighs 17 stone, could still insist that the first act of intercourse with the woman was with her consent.
She had been lifted off the ground by the gang, stripped, assaulted and then held down on the ground for the attack near King's Cross, London, last September. Afterwards she was thrown in a canal. "How you can possibly imagine, if you do, that a woman who had been so treated was agreeing to have sexual intercourse with you is beyond understanding," the judge said.
He added that the victim identified Henry as one of those who helped to push her into the canal, having heard her say that she could not swim.
Sir Lawrence said he had taken into consideration Henry's age. "But the victim's account made it clear that, although the youngest of all, you are to be treated as a leader of your gang." Last month, the other seven youths involved were sentenced to between 10 and 12 years' detention.
The judge, who had lifted an order that Henry could not be named, said the teenager should remain under supervision for the whole period of his sentence.
Henry pleaded guilty to rape only as his trial was due to start. A report indicated that he pleaded because of the strength of the evidence against him, but that he had no realisation of the awfulness of what he had done.
However, Sir Lawrence said the sentence was two years shorter than it would otherwise have been because of the plea. The 33-year-old woman still had to give evidence because another gang member denied rape.
At the time of the offence, Henry was in care and on bail for robbery, for which he was due to be sentenced on Monday. Scientific examination of his clothing revealed the woman's blood on his jacket and T-shirt, he said.
Michael Cogan, for the defence, said Henry would have liked to have had the opportunity to apologise to the victim.
The teenager had always been ashamed of his size, which had placed a burden on him, he added. "He mixes with people older than him and has to live up to their age. To a large extent he lives in a fantasy world."
The former husband of the victim watched the proceedings from the public gallery with an interpreter. The woman left him after he blamed her for the attack. He has since said he regretted his behaviour.
The judge also ordered that the identity of two more of the gang can now be revealed. They are: Timothy Davies, 17, and Colville Angol, 18, both of Islington, north London, who each received 11 years last month.