Remove the 2 bolts that hold the caliper to the caliper bracket or saddle, remove the 2 bolts that hold the bracket to the spindle. The rotor may be stuck to the hub. Carefully but forcefully, Rap the rotor between the studs with a hammer. If it doesn't loosen, whack it from behind, spin a quarter turn between whacks. Clean hub area, apply anti-seize around it before reassembling.
Clean all areas that pads (replace pads at the same time) slide on, slide pins, bushings and lubricate with high temp caliper grease.
The rear calipers almost always have to be replaced on these models because of the slides seizing or the e-brake contraption seizing. I would even screw with it, just replace it and thank the fine GM engineers for it : wink: