1) Is there an external program that we could use to edit parking spots?
Your question is not very clear: do you want to edit the GSX information, or the underlying AFCAD ? If you want to edit the GSX data, there's nothing easier and faster than the using the provided GSX editor.
2) Is there a way to change all parking spots at an airport or multiple airports? For example, I have found for a particular aircraft, 45 meters is the ideal max distance for the marshaller. It would be great to be able change that for every all parking spots at an airport(s) instead of modifying each parking spot.
Of course you can, the GSX manual at Page 24 says "Don't forget you can do this for multiple parking spot at once".
The selection dialog tree works like any other Windows selection tree: if you CTRL+click on each item, you can select multiple items. If you click on a root node like a Terminal, it will select all parkings spots for that terminal, and if you select the top airport ICAO node, all parking spots will be selected. Once multiple parking spots are selected, every change to the settings will be applied to all selected items, and a text will remind you how many parking spots will be affected.
