His contract with the NZRU and Crusaders was terminated last year due to poor fitness and he moved to France. However, he only lasted one season of a two-year deal with Clermont Auvergne to return home due to personal reasons.
Gibson acknowledged Guildford would find it tough to earn the respect of his new teammates.
"He's probably been carrying that bad-boy baggage around with him for the past few years, and from my observations he has moved past it and grown up a bit," said Gibson. "His challenge will be to demonstrate he can make good choices and prove that.
"I've spoken with Zac extensively, and he is prepared to earn the respect of his teammates through his actions, and to uphold the values of our club. Of course his signature comes with risks for both parties, but we strongly believe in his ability and will support him with his efforts to continue his rugby career.
"He's highly motivated and aspirational about his career - so our challenge will be to get the best out of him."
Guildford is likely to compete with youngster Andrew Kellaway and Matt Carraro for a spot on the wing with Wallabies squad member Rob Horne set to return to outside centre or stay on one flank.
Guildford played 11 Tests for the All Blacks. He started his Super Rugby career with the Hurricanes in 2008, playing 18 matches before moving to the Crusaders.
He made 61 appearances for the Crusaders and also won a Commonwealth Games gold medal in 2010 in Delhi with the Kiwis' Sevens team.
-AAP