Land or GDP are not gain. So exactly what has China gained ? HKese will not pay more taxes than they were paying. And if forced they will just leave (again) and take GDP with them.
You may not fire it but there is cost of creating and maintaing a gun. Yes there might be skirmishes during birth of City-as-a-Service industry. But it will stablize as military states fails to find enough revenue to sustain itself while also maintaing welfare spending.