Is there any reason why the gantry has to move on a CNC router? Seems to me you could get a lot more rigidity by moving the table along one horizontal axis instead. Then just move your Z axis assembly along the gantry for the other horizontal axis, but have the gantry solidly bolted to your table base. I realize it will take up more room when working this way, but that's no different than a mill.
Just thinking out loud.