There's an aluminum BB shell "insert in that carbon frame. They didn't face the non-drive side as it wasn't necessary. There should be enough material protruding beyond the BB area to accept some facing. Hard to tell from the pics, but it appears there is. Hard to say though since it's a carbon frame, and each one could be slightly different in that area. You'll see when you pull the non-drive side ring. If there's enough material (aluminum) have it faced.

The "facing" doesn't need to be nearly as wide/fat as on the drive side. Make sure though that the sheel width after facing is close to 68mm. GXPs are somewhat sensitive to shell width, but there are shims supplied to make up for any discrepancies and to adjust chainline. Follow the installation instructions on SRAMs site for GXP.