Crank arm play, should I get spacers?
I believe there can be an issue with play in GXP cranks. The play in the bearings is taken up by the non drive side crank arm sliding up along the splined taper as the bolt is done up and compressing the wave washer in the drive side. The bolt can be tightened up to the correct torque but there is still lateral play. 

Definitely check you have the spacers and wave washer installed correctly. You may need an additional shim to take up the play.