Mr Justice Smedley told him the sentence was "substantially reduced" because of the help he had given the Metropolitan Police. The court heard that Green's gang announced they were the Sad Posse - Seek and Destroy - before terrorising and robbing about 100 guests at the party. Green also shot one of the male guests in the foot.
He pleaded guilty to conspiracy to rob, wounding and possessing a firearm with intent to endanger life at a second trial in Leicester in June.
The first trial was abandoned after Green's role as an "A1" police informant on Jamaican Yardie gangs was revealed.
The court was told that Green came to Britain in 1991 and became a registered informer soon after.
Nicholas Gardiner, for the defence, told the court: "He was of great importance to the police because of the difficulty there is in gaining information on these organisations.
"Few people are prepared to assist with information because of the terror of reprisal".
Mr Gardiner added: "The international Yardie community will obviously be looking for him and, to put it plainly, if he's caught he will be killed."
Mr Justice Smedley said: "You took part in this terrifying robbery, weapons being fired into the ceiling, your weapon being discharged on the floor.
"That sort of appalling violence can only result in people receiving substantial prison sentences in the region of 14 years."
But he described Green's decision to give evidence for the prosecution at the second trial as "exceedingly courageous", adding: "It does mean that I shall give you a substantial discount on your sentence."