I'm glad I waited for the design sketches as I mis-understood your question in the first post, now it is clearer.

Gantry end plates at least 15mm, more like 20mm to give yourself a chance with the aluminium machining. 1500x800mm is a large machine and stiffness drops considerably as you go bigger. Really I would recommend raised X axis designs for aluminium but you here now so need to recover what you can.

Long axis (I would call it X) end plates are to stop the machine ploughing off the end of the rails but you'd be surprised how powerful the steppers as when driven through the force multiplying effect of the ballscrew. You could sit on the gantry for example and it would move you around without a problem.
I would not go for 6mm end plates, but 10-15mm as a guide. Final decision would most likely be decided on if you can cut them out of an offcut plate used elsewhere as they would be quite small. The other thing to consider is they normally double up as stepper motor mounts for the X ballscrews so that will also drive the thickness.

Z axis and Y axis plates would go for 20mm personally.