The usual idea is you panic about the gantry bending, make it enormously heavy in steel, then fit the biggest stepper motors you can buy to compensate for the weight, then make the torch lift assembly overly massive because anything else would look silly.
This seemed a bit odd to me so I took a 3 meter length of relatively light 4" box section aluminium, supported it at either end and used a DTI to see how far it bent down when I put a 10kg bag of lead shot in the middle.
10kg bent it 0.5mm so lightweight gantries are possible. The bending force is worst when you accelerate the gantry with the torch in the middle, fortunately the sums are easy if you do my little bending test.
Keeping the gantry weight down has the enormous advantage that you can use a smaller size NEMA23 steppers and you can get MUCH faster rapid transits with no loss of accuracy if you pick the right one. Moving the torch head across requires a lot less power than moving the gantry.