Works fine here. That parking has been set by the developer to have only a Left Pushback and, when I tried it, it offered me a Left Pushback and a Straight Pushback. Tried with a default 747.
However, some planes might have additional pusback constrains in their internal confguration. The PMDG 747, for example, has been set to prevent pushback if you still have the wheelchocks. GSX will of course tell you that but, if you move the Verbosity slider to the lowest settings, you'll miss any warnings from GSX.