Find the following file (which will be a hidden system file) 4d5308d2.gpd
then delete it & when you logon back to GFWL it will let you re-earn it. Obviously make sure your not in the game when you do this just be on the Windows desktop.
The .gpd file is just a temp GFWL achvs marker downloaded from GFWL servers & no matter how often you delete it whenever you logon back to GFWL it will get redownloaded to your PC.
Its there someone on your hard disk it may be hard to find but every GFWL game has a different .gpd file. One thing to mention is whenever you delete the .gpd file & you first logon back to the game ALL achievements will appear locked in the GFWL client but as soon as you reearn any achv they will all magically unlock again as they were before.
Nothing you can ever do to the .gpd file will affect any unearned achievements on the GFWL/Xbox.com servers either!!