The St. George-Illawarra captain, the nephew of rugby league "immortal" Reg Gasnier, announced the switch in a news conference on Tuesday, explaining it was because a 2006 deal to keep him in rugby league had not been fully honored.
Gasnier said he would not have entertained offers from rugby union had his five-year contract been honored, but he had no choice but to leave because he felt like he was only earning "two-thirds" of his wages.
He said he activated an exit clause in his contract because he was owed third-party sponsorship money promised to him when he signed two seasons ago.
"The hardest things was to walk away from the club, I love the club," Gasnier said.
"I grew up here, there's a lot of family history.
"But it's the circumstances that led me here.
It's unfortunate how things come about but I've had to react to it and I think this is the best reaction for me."
Gasnier, who will now work with former New South Wales Waratahs coach and Wallaby prop Ewan McKenzie at Stade Francais, will remain as St George-Illawarra captain for the remainder of this National Rugby League season.
McKenzie worked with high-profile rugby league converts during his tenure at the Waratahs, including Lote Tuqiri, Wendell Sailor, Matt Rogers and Timana Tahu, before his contract expired following a Super 14 final loss to the Crusaders in May.
Gasnier, who turns 27 on Saturday, has played 133 NRL matches since his debut in 2000 and 15 rugby league tests for Australia.
A tall, powerful midfielder with pace, a strong fend, good swerve and versatile kicking game, Gasnier is regarded by McKenzie as the ideal recruit to the 15-man code.
However, it is unlikely he will play for the Wallabies - Australia's national rugby union team - in the near future after ARU chief executive John O'Neill confirmed Monday that only domestic-based players were eligible for selection.
