So I guess GSX does not trigger correctly the pushback state in p3d.
Of course GSX doesn't trigger the default pushback, and it would be wrong if it did, since the default pushback, once triggered, will move the airplane together with GSX, so the two systems would fight against each other.
So, GSX is entirely correct not triggering the default pushback, and your problem is that your Prosim addon is, instead, only compatible with the default Pushback, something they could fix easily, if they just read the GSX Manual ( GSX can be freely downloaded at any time, so they won't have to buy it ), which has a "Developers" section in the last page, which is how other developers like FS Labs, Aerosoft, Leonardo, FS++, etc. obtained all the informations they need to interface with GSX.
In this case, at Page 68, the paragraph named "Checking the Pushback status", would reveal that, for any add-on wanting to know when GSX is starting to push, it would be enough to check the following variable:
L:FSDT_VAR_Frozen
Which is set to 1 when the pushback starts, and goes back to 0 when it stops.