That's a good start, but at first glance, the gantry looks a bit tall for the size of the supporting columns. Since you're using sheet metal a U-shaped piece could be added to each column - similar to what you have done, just bigger. This would increase the width of the machine, but add a lot of stiffness.

What is it you're planning to use as the base of the rails for the bed? In the drawing it looks like just a thin metal sheet, which I'd advise against as it's unlikely to be sufficiently flat for the rails to perform well.