I don't know about the Aerosoft version but, default LOWI has Pushback set to "None" in all Parkings, only the Gates 1, 2 and 3 have a Pushback set to "Both".
GSX respect the Pushback preference set in the AFCAD so, if you tried Pushback at non Gate location it's normal you would see that message. Perhaps there is a reason why the Pushback has been set to None, in some places it might not be allowed as in real life. But also the designer of the might just have overlooked it, but GSX can't obviously know the difference.
We are working on an update that will allow you to customize parkings, overriding what's in the AFCAD.
For the time being, you have to modify the AFCAD yourself. Unfortunately, ADE doesn't allow to set Pushback preference easily (you'll have to save the file as XML, modify the Pushbacks in the XML and compile it with the BGLCOMP from the FSX SDK), the AFX program from Flight1 allows to set Pushback preference very easily.