I tried it just now, and I can't reproduce it.
When pushback is requested 50 procent of the time I have the option nose left or right etc. The other times there is only 100m backwards or Quickedit. Why is the nose left or right not always available ?
It's not obviously "random", it depends on the Pushback preference set in the scenery, the manual explains that, Page 29:
Straight Pushback Distance
Some sceneries have all their parking positions Pushback preference set to “None”. Even in this case, GSX will always offer at least one choice of a “Straight Pushback”
Having said that, are you noticing this problem everywhere ( meaning when you DO have Left/Right available), or just when Straight pushback and Quick Edit are the only available options ?
You can change this, either by changing the Pushback preference in the airport customization page for a specific gate on a specific airport, but there's also a global setting you can change if you want to always have Left/Right available, even if the scenery set it to None, explained at Page 31:
Always ask for Pushback
This option will enabled Pushback, globally, even if the Pushback preference in any specific airport has been set to “None”, and will always assume “Both” directions are always available