Commenting afterwards, Ireland coach Eddie O'Sullivan said: "Gordon suffered a stress fracture of his forearm. That's the end of his Six Nations and he will have an operation on Monday."
It is desperate luck for the Leinster centre who was hoping to hit the ground running in the championship and show the sort of form which won him the Six Nations Player of the Tournament in 2004.
Meanwhile, Paul O'Connell is progressing well with his recovery from the back injury he sustained during the World Cup in September.
He has not played for Munster or Ireland since then, but the talismanic lock is targeting Ireland's next home match against Scotland on February 23 for his return.
Asked about the 28-year-old's current condition, O'Sullivan admitted: "I'm careful with Paul because he was doing well in the build-up to the Six Nations only to wake up one morning in big trouble.
"It's one of those injuries that can act up but now his rehabilitation is going well. The plan is for him to play maybe half a game with Munster 'A' against Ulster 'A' next weekend.
"The week after he'll play in the Magners League and if he gets through that he'll come into the frame for the Scotland game."