Gallen, who four-time world champion Danny Green believes could make the grade as a professional boxer, won a split decision against Messam over three two-minute rounds.
Rugby league also won the second battle of the codes bout with Gallen's international teammate Sam Thaiday beating Chiefs prop Ben Tameifuna.
Last year 32-year-old Gallen easily dispatched former All Black Hika Elliot at the Fight for Life event, but found things tougher going against Messam who had beaten Wendell Sailor in 2011.
The 29-year-old All Black found it hard to tag the Australian until the final round when a right hand found its mark to rock the Cronulla captain.
But Gallen steered clear of the knockout punch Messam needed to win the bout after the flanker was outpointed in the opening two rounds.
"He keeps coming forward ... I couldn't really hit him," Messam said.
The fight was the showcase of the event, raising funds for children's charity KidsCan, but earlier Kangaroos international Thaiday gave away more than 31kg and six years to Tameifuna who had broken the jaw of a sparring partner in training.
The 28-year-old Broncos forward avoided that fate and the judges ruled unanimously in his favour despite being caught by some stinging shots from the 144kg Tameifuna.
"It's the toughest thing I've ever done and I have a new-found respect for boxers," the Rugby League World Cup-winner said.
In the battle between former international cricketers, Chris Cairns put the controversy of being the subject of an investigation into alleged match-fixing by the International Cricket Council to one side to beat Simon Doull in a unanimous decision.
Cairns, who lost 20kg in shaping up for the fight, alluded to the probe after the bout but it was Doull who made specific mention of the allegation that came to light last week.
"Innocent until proven," he said.
Sky television presenter Stephen McIvor and Fairfax journalist Steve Kilgallon drew the opening bout of the evening.