Hi Leo, the GatewaySubnet learns all other subnets of its own VNET and all subnets of all peered VNETS and those subnet address spaces are added to the GatewaySubnet system routes automatically. You can check this by viewing the Effective routes of the GatewaySubnet route table. So you need to overwrite those routes with your user routes. To do this you need to add same or more specific routes. The /23 is a larger route than the /24. So the /24 system route will take priority.
in short, yes you need to add the specific /24 routes.