There's nothing in need to do to see the Marshaller, other than selecting a gate after landing.
The Marshaller readouts ( the numbers ) are controlled in the GSX Settings but, no matter if you have the readouts enabled or not, the Marshaller itself will always show up. Of course, provided it hasn't been explicitly disabled in a custom GSX airport profile for a certain airport.
Another possible reason of the Marshaller not being visible, is that the parking spot is too close to a building, so the Marshaller is there, but inside the building. Which is why, GSX offers several ways to configure both the Stop position and the Marshaller position in the airport customization.