It's strange you would prefer going back to the default Pushback, which is the service where the distance between default services and GSX is the highest one. If it was "just" for using voice control, what about using something like Voice Attack to control the whole GSX menu in remote control ?
In any case, there's no way to have GSX remove all default ground vehicles *except* the Pushback, and we are not planning to do that. We'd rather add voice control in GSX, instead...