Explanation
A valid insertion point for North-South network introspection is Tier-0 gateway. North-South network introspection is a service insertion feature that allows third-party network services to be integrated with NSX. North-South network introspection enables traffic redirection from the uplink of an NSX Edge node to a service chain that consists of one or more service profiles1. The Tier-0 gateway is the logical router that connects the NSX Edge node to the physical network and provides North-South routing and network services2.
https://docs.vmware.com/en/VMware-NSX/4.1/administration/GUID-D5933474-34A2-4DCE-AE9B-A82FF33E