The output gap in numerical terms is the difference between the size of demand for the existing goods and services and the existing production capacity to supply those goods and services. However, in reality, people’s demand itself has been changing due to structural changes in society, including aging.
If the supply side has not been able to sufficiently meet such new demand, then it might not merely be the problem of an output gap but the problem associated with a mismatch between demand and supply, namely, demand has not been meshing well with supply.