Does the surgery for GERD focus on the stomach or the esophagus?

Both. The problem with gerd is often the junction between the esophagus and stomach, the lower sphincter, is inappropriately in the chest, allowing acid to reflux into the esophagus. A nissen fundoplication wraps the redundant fundus of the stomach around the esophagus, and brings the sphincter back into the abdomen; this creates a valve to limit acid reflux.
Both. Reflux surgery involves repairing the valve at the junction of the esophagus and stomach. This is usually done by wrapping the upper portion of the stomach either partially or completely around the distal esophagus.