Crossing from Galtat Zemour to Bir Mogrein is impossible, the Morocco army doesn't allow it at all. The other way is possible, Mauritanian army let you pass, but very difficult to proceed some kms ahead since the Polisario have established a permanent barrac in the middle of the track.
About the Dakar track, it really doen't pass at GZ - Bir, but at the north border (Smara - Amgala - Bir).
This was the situation in February 2006 but it seems it remains the same. I have a friend that will pass at GZ next week, I'm gonna ask them to check the situation there, however the situation should be checked at Bir, since at GZ for sure it remains the same.