Gasnier yesterday announced a two-year deal - worth $1 million a season - to play rugby union with glamour European club Stade Francais, at a press conference at St George Leagues Club.

The Dragons captain said he could not see himself coming back to play rugby league in Australia, but the Mercury can reveal he has provided an assurance to his club that he would never turn out with a rival team if he did return.

"Gaz has guaranteed me he would play with the Dragons if he came back," Doust told the Mercury last night.

"I won't be commenting on specifics, but I would like to think that he would never play against the Dragons."

Gasnier, who was contracted with the Dragons until the end of 2011, is leaving the club after using a get-out clause in his deal because of a massive shortfall in third-party sponsorships.

The decision means he will miss the Rugby League World Cup played in Australia in October and November.

With Dragons fans venting their fury over Gasnier's decision to quit, Doust called on supporters to back their captain as they chase a place in this year's finals series.

"It's a sad day for the Dragons, and rugby league," he said.

"But Mark has shown honesty and sincerity as he has explained his position, and he will be busting his butt for the team for the rest of the season."

The Dragons have two remaining home games at WIN Stadium, against Wests Tigers and the Warriors.

Gasnier will swap the famous Red V of St George Illawarra for the pink jersey of Stade Francais as soon as the Dragons' promising season is over.

The NSW and Australian centre said he was hoping to have a long-term future in French rugby.

While Gasnier would not rule it out, he said he could not see himself returning to the NRL.

"It's tough to see the future, but I don't think so, no," he said.

"You never say never, but if I had to give you an answer now, I'd say no.

"I hope to try and go well at it, and if that's the case, sign on for longer."