 Solution

H is the fused pyridone analogue of G: replace the pyrone ring oxygen by NH, retaining its carbonyl and the remote carboxylic acid. Ammonia adds to the electron-poor pyrone system, opening its lactone/enol-ether framework into an amino-containing unsaturated acyl intermediate. Proton transfers permit intramolecular closure through nitrogen and loss of water, giving the conjugated pyridone/lactam ring. Acid work-up restores the neutral acid and the preferred neutral lactam representation.

The net atom change is $+\mathrm{NH}-\mathrm{O}$, soexactly matching H. Merely forming a carboxamide from the terminal acid would remove a different oxygen and leave the pyrone oxygen intact; it would not explain the characteristic pyrone-to-pyridone ring transformation.