The 31-year-old forward joins after leaving Salford Red Devils, and should be with the Centurions until at least the end of the November 2016.
There is also an option for the club to extend the deal by a year if things work out well.
“Upon learning of Gareth’s departure from Salford I was shortly contacted indirectly from Gareth with a strong desire to become a part of our team,” said Leigh head coach Paul Rowley.
“Rugby League players of Gareth’s ability rarely become available and there was an obvious interest from Derek and myself to research and exhaust an opportunity to add a world-class player to our team.
“The staff and my players are all very enthused and excited to welcome Gareth to the family and challenge him to work hard and express himself through his talents, whilst also challenging myself to get the best out of him within a very positive, fun and hard-working environment.
“I am looking forward to watching him represent this great Club with the same pride and passion as that of all our players.”
Leigh head of rugby Derek Beaumont is delighted that his club have acquired a player of such quality.
“Gareth is a top quality rugby league player with a proven track record,” he said.
“It was evident to me in the performance for Salford against Saints that he was becoming frustrated.
“He is a born winner, a tough individual who leaves everything on the pitch.
“If Gareth was an individual sportsman like a boxer or sprinter where he was totally in control of the outcome he would be a World Champion, such is his desire and commitment to win.
“However, he participates in a team game and when that appears to be letting him down his enthusiasm to make up for others can run away and that is something he won’t experience here at Leigh.
“Gareth comes to us with a complete clean sheet of paper upon which he can write his own future. He will be judged and rewarded by what he does at Leigh, not what he has done in the past.
“He has been brought to our club because he is a world class rugby player and that is all I am interested in.”