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GXP cranks - in my experience - can only be adjusted with spacers on the NDS to pull the DS inboard. Spacers on the DS, beyond taking up any space between the shield and the crank spider*, cause the NDS arm to bind the NDS bearing.
* space, not slop, because GXP cranks when installed properly have no slop regardless of shell tolerances. The whole point of GXP is that the crank bolt is only used to attach the NDS arm to the axle, and plays no part in bearing pre-load or removal of horizontal slop.
Howard-
GXP cranks when installed properly have no slop regardless of shell tolerances
This is right. Crankshaft end float on GXP should not happen, because the NDS crank traps the NDS bearing against the step on the spindle. That's the whole reason why they have that annoying difference in bearing bore between DS and NDS.
gbj_tester
You can use spacers to adjust the chainline with those crankset no problem. But indeed you have to tighten it only to the point the crankset stop any horizontal play (as specified in the SRAM manual you get with it new). More and you crush it as howard explain.
There is a tolerance build in the crankset for the bearings to seat in a wider area than necessary so few mm spacers can't do any damage. Remember that chainline is being adjusted with spacers on the non drive side to pull the crankset toward the frame.