I'm not Umberto, but it sounds to me like its a problem in the original AFCAD which would need to be edited. Since editing AFCAD the are bgl and I don't have that AFCAD file, the way around that is to edit the position using GSX itself. In P3D, along the top bar, open the Addons menu, then GSX, then Customize airport positions. This will open the customization window. Assuming you are already in the aircraft you want to have at the parking position, click the airplane icon next to the position size - this will resize the position to be able to accommodate your aircraft. Then click on Edit parking position. This will close the edit window and you will be back in the sim in Slew mode with a bunch of lines and circles. You'll be starting off editing the parking position itself. You can use the arrow keys on the keyboard to move the position forward, back, left, and right. The other keyboard shortcuts for rotating and diagonal movement are in the edit window. When you have the position where you want it, push F4 to go to the next item and repeat. GSX will save the data so you don't have to do this again.
Please note however, this is only changing the data for YOUR aircraft. Meaning if AI parks there, it will park where the original AFCAD says it should be. Also if you load up the sim into that spot, it will be where the original AFCAD says it should be and you'll have to slew the aircraft into the correct position to start. The only way around those is to edit the original AFCAD, but if the bgl file has ANYTHING other than the taxiway and parking data in it (like scenery objects), those may be lost when recompiling after making the changes.
