This issue has been discussed many times already, and of course on the GSX manual too, Page 23. GSX reads the information from the AFCAD so, if the AFCAD designer set the pushback preference to "None", GSX can't obviously decide if it was really meant to disallow pushback (this might be needed at some places) or simply because the scenery designer forgot to set it.
Which is why, there's a complete scenery customization feature in GSX, which allows you to override many settings in the scenery AFCAD without having to modify the scenery. GSX Manual, chapter aptly named "Airport Customization", starting from Page 22.
