This has been discussed many times already, for example here:
http://www.fsdreamteam.com/forum/index.php/topic,6097.msg53872.html#msg53872and of course it's also discussed in the manual, and of course the previous user already replied to you with the reason.
GSX doesn't know anything of the outside scenery, other than the AFCAD. It doesn't know if there's a building or another obstacle. The Marshaller "ideal" position is calculated automatically depending on your eyepoint position inside the cockpit, but sometimes this distance is too high, because there might be a building too close behind him, so he will be placed inside the building and you won't see him.
WHICH IS WHY, GSX has a Parking Customization page, where you can specify, for each parking, several parameters, and the one you need is the "Max Marshaller Distance", so you can adjust the calculated distance to be not greater than a certain amount, so the marshaller won't end up inside a building.
Note that, the calculation for the distance use a lower value, if the parking has a jetway, on the assumption that parkings with jetways have a building behind that *might* cause problems, while parkings in the open usually have plenty of space for the marshaller. This means, if you are using a scenery that has fake/static jetways that cannot be detected automatically, sometimes might be enough to just enable the option "Parking has a jetway", to have the Marshaller reappear without having the change the Max Marshaller distance.
Doing this is worth in any case, because if you have a parking with a fake/static jetway, it's best to tell GSX about it, so it won't call for a passenger bus and front stairs.
