I don't get the part on API gateways. Gateways are north south most of the time while a sm is east west. They can work together perfectly fine and don't even have to be integrated.
Supporting North/South is how they have traditionally been marketed, but not how they are actually used much of the time. Inside enterprise they are often used as an internal "service catalog" and are effectively a shim providing discovery and consistency over a bunch of fairly scoped internal services.