Check if the pushback availability is set to "both".
More precisely, check it hasn't been set to "None". As explained on the manual, GSX reads the AFCAD, but it can't possibly know if the scenery designer set it to None because he really meant to ( some positions might not allow pushback ), or it was just an oversight. Of course, this can be customized in the GSX scenery editor, so you can override what the original designer specified in the scenery AFCAD.
So, if the parking says "None" to Pushback, it's just normal you won't see any truck, and pushback will not work there (but the FSX default pushback will, since FSX ignores that setting).
If, instead, is set to Left, Right or Both, and you are missing the truck, there might be a problem.