I don't see why you would want to cast a gantry. The open back structure significantly limits stiffness.

If you compare it to a simple 80x160x4mm steel RHS section you would need to make the cast aluminum 'C' shape gantry have web and flanges of 60mm thick to match the bending and torsional stiffness.

Makes much more sense to cast the end connection down to the bearings blocks and ballscrew, with a neat interface to say a 160x80 RHS steel gantry.