What does "severe'" gas-bloat after nissen fundoplication mean/look like? What causes gas-bloat after this surgery?

Can't burp. An effective nissen fundoplication provides a barrier to gastroesophageal reflux, but also a barrier to vomiting and burping since the lower esophageal sphincter no longer relaxes to let air (or other stomach contents) escape upward from your stomach. The air then accumulates in the stomach, resulting in gas and bloating until it can be evacuated into the small bowel.