As explained, many times already, GSX can't decide automatically if the Pushback preference was set to "None", because the scenery designer really *meant* no Pushback should be allowed on that parking, or simply because he made a mistake/forgot about it.
The "Customize this parking position" option in GSX is there to fix this, so you can override the information found in the scenery AFCAD without having to modify the scenery, and of course is clearly explained on GSX manual, Page 23, the chapter aptly named "Pushback Direction"
GSX reads the Pushback preference from the AFCAD file, which can be Left, Right, Both or None. Normally GSX will not allow pushback from a parking position that has its preference set to None, on the assumption the scenery designer had a spe- cific reason to disallow Pushback on that parkign spot. But, in case the AFCAD is simply not accurate, this option will allow you to instruct GSX to override the Push- back preference set in the AFCAD with the one specified here