Why a Gearing could not simply take the 5-inch/54

Could a Gearing-class destroyer take the 5-inch/54 Mark 16 gun?

Not economically; the necessary rebuilding would make a new ship the more sensible investment.

The panel's short answer is no. The 5-inch/54 Mark 16 is not a drop-in replacement for a Gearing-class mounting. Adapting the old destroyer would require extensive and expensive rebuilding around the weapon and its supporting systems. Once a modernization reaches that scale, new construction offers a better return, which is the path the United States effectively followed.