You need a pretty stiff machine to cut steel and the fixed gantry is a good start if the parts are small. Unfortunately using 10mm plate on the Y axis and the gantry sides might not be enough.

I would substitute the 2off Y axis plates for box section which would stiffen up both the Y axis and the gantry sides in one go. Ali profile would be easy to fix by tapping the ends and bolting through the gantry sides. Or you could use steel box welded to small plates which were in turn bolted to the gantry sides (bolted allows for alignment).

For the base I would consider using box section all round instead to give the gantry sides more support, or if you want to stick to plate at least closing off the front of the machine with more plate to stop the sides bending in.