Sry dude, but I did some searching and it looks like you might be screwed. The achievement is locked to your GT and the game does not have the same work around that it did last time. I will keep trying and looking. If I find any news, I will let you know.
Sadly... I have seen it happen a lot and right now I am pretty sure there is no fix for it. When I finally get around to playing the game I will start to try some methonds. You could be in luck Did you connect to live after you unlock that achieve or what. If you havent. Oh wait let me just go ck your card.