How many spacers have you fitted? There should only be 2 for an 68mm shell. The GXP system relies on you tightening the bolt up a lot, as jonathon says try removing it, re-greasing and tighten again. This will pinch the non-driveside bearing between the step in the axle and the crank arm. I think this may possible leave a gap between the seal and the crank arm on the driveside but this is ok. There is no need to pre-load the bearings like on HTII.
I used to have some pics showing this as I had a similar problem the first time I fitted a GXP but I can't find them at the moment.