Hi Jonny.
I like it. Looks pretty rigid.
Frame is awesomely triangulated, carriages on your long axis are well mounted and gantry ends are well re-enforced. I like the raised X rail design (I am building similar).
Personally, I don't like t slot extrusion for a gantry, it is comparatively squishy when compared to rectangular tube of suitable wall thickness, especially if you include diagonal bracing internally. Rails mounted over T slots also make space for flex, as the t slot limbs are unsupported cantilevers, and it still requires machining to be suitably flat for good linear profile mounting.
I cant see how your ACB's are mounted, but consider gussets on their mounting flange to account for axial thrust, the cantilevered mounting plate can flex.
Are you machining your X rails for the linear rails? Hot rolled tube is not nearly flat enough for linear profile rail, you also need to be able to get the rails level to each other very accurately (though having a flexible gantry can account for some misalignment).
Z axis plate will let the machine down in terms of stiffness, consider flanges on the sides to give increase its stiffness in its weak plane.
any reason you selected bearings on top and bottom of your gantry instead of in front? The increased bearing spacing will improve joint stiffness, and overhang, but you sacrifice Z clearance. This may not be an issue if you are only planning on doing flat sheet work...
An additional spindle bracket, mounted on its upper half, will also improve that joint stiffness, a lot.
My post reads very critically, I don't intend it that way, your machine looks awesome, and will likely cut wood awesomely and Aluminium OK as well. Lots of people gave me great input when I shared my design intention, and many of these topics have been discussed in this forum in great detail. I will be following on with interest
Luke