you put one 2.5mm spacer on each side. It's on the poorly written documentation that comes with the GXP BB package.
or it should be like that, at least. The manual is poorly written, like I've said. I had to use one 2.5mm spacer on each side with my truvativ touro triple with a 68mm shell.
only use spacers for mtn crank. the spindles are different lengths between mtn and road. not so much poorly documented but the bb is the same between road and mtn