The difference in Q-factor (width between the pedals) between SRAM GXP and BB30 is like 1mm or something. They are very close.

The BB30 crank is narrower at the bottom bracket, which makes it stiffer. But crank arms on the BB30 version flare out more than the GXP version, so they end up being about the same width at the pedals. I have SRAM Force cranks in BB30 and GXP, and it's a visually noticeably difference in crank arm flare out between the two. The GXP is much closer to straight up and down, while the BB30 flares out.