Looking good so far. You could use the machine to make reinforcement plates to brace the gantry (assume you don't have milling machine etc) as that profile is not as stiff as steel and it looks like you are using simple fasteners to hold it all together.

Once you get the weight of the y and z axis fitted and the spindle give the gantry a thump with a soft hammer and you will feel it vibrate. just something to think about as you progress.