There is a reason why even a straight pushback option is always available, even when the pushback preference is set to "None", and it's because many 3rd party sceneries are still released with "None", because older versions of ADE didn't allow to edit that setting, because it was ignored by AI.
GSX cannot possibly know if a scenery developer set the pushback preference to None because of a mistake, or because he really meant *no* pushback is allowed there and, since the number of 3rd party sceneries in which the pushback preference was set to None by mistake was surely higher than the few cases in which there was a legitimate reason to prevent pushback there, many users assumed GSX has a "bug", because when the scenery set "None", it didn't offer any pushback. This happened years ago so, following many users requests, we added the option to always have a Straight pushback at least.
However, since many users wanted to set a gate with a real "no pushback" option, we added the QUESTION: when the pushback preference is set to "none", you are being asked if you want to perform pushback or not, and this is of course the only sensible course of action because it will still give you the flexibility to still have pushback there, if needed, without have to re-edit the parking position again, just because you needed it once.