Just gonna mention it again as it seems to be of utmost importance to getting decent life out a gxp B.B., face the shell!
Also, slightly poor info was given upthread, an ultegra B.B. will not work as a replacement. Sram cranks have a different B.B. spindle diameter on either side so you need a B.B. with appropriately sized bearings. Ultegra (or any Shimano) would have the same size either side.
Genuine gxp bbs are a bit dearer than Shimano (and the other options would be Hope and maybe Phil so £££) but face the shell and you shouldn’t have any problems with them wearing out prematurely.