The Irishman, a summer signing from Coventry, faces an unknown period of time on the sidelines following the incident on George Street at 3am on Sunday. A Hibs spokesperson said:
"Gary was the victim of an unprovoked attack at the weekend and has since had surgery.
"It is not clear at this stage how long he will be out for but it will be a number of weeks."
Lothian and Borders Police appealed for witnesses over the serious assault. They said:
"A 25-year-old man had been within the Le Monde nightclub and was standing near the Hardrock cafe when a male approached and punched him to the face.
"The suspect then made off and the victim also then left the area with friends.
"An ambulance, which had been called by members of the public who witnessed the attack found the injured man on Rose Street and he was taken to hospital for treatment on a broken jaw."