Mustapha Khaznadar, 25, was extradited to the territory last month from NSW to face drug and firearm charges from 2010.

On Thursday Khaznadar pleaded guilty in the ACT Magistrates Court to four charges, including causing public alarm, and possession of cannabis and cocaine.

Court documents said patrons at Belluci's restaurant were forced to duck for cover after an armed Khaznadar walked up to the window about 7.50pm on May 18, 2010.

He first made hand gestures and mouthed the words ''Come outside'' to a patron seated near the window. He then lifted his T-shirt to reveal the handle of a firearm before pointing a sawn-off shotgun at the man and mouthing the words: ''You're dead''.

The defendant then fled in a car waiting nearby.

One witness reported seeing the man who had been threatened make a phone call soon after, and heard him say in Arabic: ''I'm not scared of you, I don't [care] if you're at my house, get back here.''

Court documents said patrons reported being in fear of being seriously injured and described pandemonium at the restaurant as people tried to get out of the line of fire. The gun was not discharged and no injury was caused during the incident.

A number of witnesses told police the incident left them shaken, unable to sleep at night, and suffering shock for days. Court documents said some patrons sought counselling over the incident.

Khaznadar fled interstate before the matters could be dealt with before the Canberra courts.

In February 2011, a NSW District Court judge sentenced Khaznadar to more than five years in jail for weapons offences.

He was returned to the territory to face the charges after he was granted parole last month.

It is unknown if Khaznadar was associated with the Brothers 4 Life gang at the time of the 2010 incident.

The western Sydney street gang became notorious in recent years after an internal war boiled over into a series of public tit-for-tat shootings and murders.

Canberra Magistrate Robert Cook will sentence Khaznadar for the Griffith offence in May.