Isn't the axle length the same, you are just spacing the BB on a 68mm shell?
Yeah that's what I'd originally thought (hoped!) but the evidence does seem to be pointing to not being compatible. The SRAM Rival 1 product page suggests only 68mm BB compatibility but not sure if I'm reading it right. But also that page says the spindle diameter is 30mm but when I measure mine it's 24mm, so maybe I'm not looking at the right thing.
you may as well get whatever BB suits the new cranks you want rather than trying to match to gxp.
Yeah that makes total sense, thanks. I guess given the rest of what I've got is SRAM (X5 10spd derailleur and X9 shifter) I'll probably stick with SRAM cranks for the sake of harmony!
Thanks for the input all, really helpful 👍