This is really a FAQ, which has been discussed many times already on the forum, and of course is explained on the GSX manual too when airport customization is discussed, the chapter named "Pushback Direction"
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 specific reason to disallow Pushback on that parking spot. But, in case the AFCAD is simply not accurate, this option will allow you to instruct GSX to override the Pushback preference set in the AFCAD with the one specified here.
With earlier GSX versions, Pushback at these places simply won't start but, since we got so many reports from users that there are many sceneries out there where the parking preference has been set to "None" ( I suspect it's because the freeware ADE program many use to create AFCADs defaults to that), in later version, we added the option to always get a straight pushback option, which explains why you only get that one.
With the GSX parking customization editor, you can set the affected parkings (those that have "None" set for Pushback direction) to be the correct value, which would usually be "Both".
