When the machine is cutting parallel to X (with Y being the gantry axis) the cutting force applies a moment to the gantry, which will twist it, hence the gantry must be a structure with good torsional stiffness (in the XZ plane). The torsional stiffness of a beam is only good if the section is closed, so for a C section it is very poor. The same is true for an I-beam or plate, so I suggest changing it for RHS or multiple pieces of box section.

Torsion Equations