I agree with the good overhead ventilation posts. Can you check your hen by cupping her lower belly between her legs for any enlargement, fullness, or tightness? Has she been laying eggs recently, and were they normal eggs with hard shells? Can you hear any rattling or wheezing when you listen to her breathing? Do you see any watery or foamy eyes or nasal drainage? Is she overweight?
Gasping can be a sign of respiratory diseases, but also might be a sign of ascites or water belly, a fluid accumulation inside the abdomen. That can happen with heart failure and some reproductive disorders.