It's likely the marshaller is there, but inside the building, since the terminal is very close to the parking. Also, GSX normally place the marshaller closer if there's a jetway, because usually there's also a building behind but, if the scenery has fake/static jetways, GSX can't obviously knows about them, so it can't use the different placing method.
You can fix this by using the "Customize this parking position" option when parked, and setting the Max marshaller distance, until you find the highest value that will allow the marshaller to be seen.