SRAM GXP bottom brackets can be user serviced, but it is usually unnecessary.
To service your BB bearings, first remove the crank arms. Then place your thumb in the BB spindle hole, and bend the joint enough that your knuckle makes firm contact around the entire ring surface of the spindle hole. Pull outward using fairly firm pressure, and if necessary, rock your thumb side to side. This will pull free the outer, plastic shield, and expose the inner rubber bearing seals.
Then take a scalpel, or a sharp awl, and use the tip to gently pry up the edge of the bearing seal. You may need to lift it at more than one area around the edge, but it should pop free with a little force.
Once the seal is removed, clean and degrease the exposed bearings, replace the grease with fresh grease, then replace the seals. Repeat on the bearing on the opposite side of the BB.
One word of warning: while this is possible, and not likely to cause any issues, it is not intended by the manufacturer, and will not be covered under warranty, should you damage the BB in the process.