In GSX options there is a setting "Always ask for pushback" make sure it is not checked and it should never ask you, it never asked me yet, although I always need it, i just do not want them to ask me for it.
Push back truck might still show up, but when you ready to depart and done with GSX services I think it will disappear with everything else as you close the GSX menu.
However unchecking this option should stop them from asking you this, I have never been asked yet.
Hope that helps,