If it was me i’d ditch the gxp and go with standard Hollowtech II stuff because i have not heard anything good about the gxp bottom brackets but you pays your money you makes your choice and go with what you want.
A quick look in the chris king website shows this
http://chrisking.com/bottombracket
COMPATIBILITY
We offer our bottom bracket cups in combination with a plastic center sleeve and three anodized 2.5mm chainline spacers for 24mm crank axle systems such as Shimano Hollowtech II. Only 108 grams.
Also available is the SRAM-compatible 24mm stepped axle configuration. The stepped axle Chris King bottom bracket makes use of a fit kit to match the crank axle to our standard cups and bearings but without use of chainline spacers. 125 grams.
So you could upgrade the BB to a Chris King then if you change the cranks at a later date you could remove the fit kit and re-use the BB.