The Grey Gatsby faces the stiffest test of his stamina to date when he lines up in Sunday's Grand Prix de Paris at Longchamp.

Kevin Ryan's charge shone over 10-1/2 furlongs (2100m) in the Prix du Jockey Club at Chantilly last month, backing up his previous mile-and-a-quarter (2000m) verdict in the Dante Stakes at York.

The son of Mastercraftsman will be stepping into the unknown, however, as he shoots for Group One glory over a mile and a half, with Ryan believing his charge will stay the distance.

"He wasn't stopping last time, it was a hands and heels ride," Ryan told At The Races.

"Ryan (Moore) didn't have to hit him, so I think there's a good chance he'll get a mile and a half. It's a difficult race but I think the big factor is how easily he's switching off in his races.

"After this, he'll go back to a mile and a quarter, and the Juddmonte's the plan."

Moore will again be in the saddle when The Grey Gatsby faces 10 rivals including the John Gosden-trained Marzocco.

He finished second in Derby trials at Epsom and Goodwood but skipped a Classic date in favour of a run at Royal Ascot.

Gosden felt Marzocco did not quite stay the two miles when third in the Queen's Vase.

"I don't think he stayed in the Queen's Vase and that's why we are bringing him back to a mile and a half," he said.

"He won't mind the ground. It's a nice little race for him and I think it's quite open."

Jean-Claude Rouget's Prince Gibraltar is the likely favourite after finishing a luckless third to The Grey Gatsby at Chantilly.