When i arrive at the gate, i can't see an marshaller because he is in the wall... How to fix this problem?
By reading at Page 24 of the GSX manual, where the "Max Marshaller Distance" scenery customization parameter is explained.
Note that, as explained in the manual, the marshaller distance calculation will change if the parking has a jetway or not but, if the scenery you use (you don't say which scenery is), has static/fake jetways, GSX cannot detect them so, in this case, it MIGHT be enough to set the "Parking has jetway" flag (assuming you *are* parking on a gate with a jetway), to change the calculation and use a smaller value. If this is not enough, you will then have to input a smaller Max Marshaller distance.