Is it normal for my stomach to have times where it is hard as a rock three months post ventral hernia surgery and four weeks post gallbladder surgery?

See a doctor: Very hard to tell here without a physical examination. Are you describing hardness around the surgical sites? If mesh was used for the hernia surgery then deep scarring firmness can be felt for up to 6 months. Are you describing gas pressure? Diet changes & avoid fatty foods for a while. See your doctor or surgeon for an exam for more help.

Seroma: After a ventral hernia repair you can get a fluid collection above the mesh between the skin and the hernia repair. The fluid collection called a Seroma, can take several weeks to months to resolve. Even in a Lap Chole you can develop a seroma at the umbilical incision. You should see your surgeon so they can exam to r/o any other causes.