Are you sure you cannot change the setting ? There's a bug in Windows 7 (which has been fixed in Windows 10, not sure about
, that a dialog on top of a DirectX Window is not displayed correctly.
Try placing P3D in Windowed mode, so it doesn't take all the screen, and drag the GSX scenery customization dialog outside the P3D window, so they don't overlap, and see if you can change the parameters.
In any case, this doesn't seem to be the problem, both because the Pushback preference is already set to Both (and if it was to "none" you would still see an option to have a straight pushback)
Could you please indicate more precisely what do you mean with "I can not run the Pushback" ? Do you see the pushback menu Left/Right ? Have you selected other operations in GSX before ? Does it work if you select "Restart Couatl" and choose Pushback directly ?