[wendtmk]

It really isn't an outboard load. As a rigid structure, the gantry could be virtually anywhere and fastened to the 4 blocks and still be considered a vertical load. Gravity will still push straight down on the bearings. Not sideways.

That would be true only if everything were "perfectly" rigid. But it's not. It will flex, and cause a rotational load, if the mass is outside the bearings. There's a moment arm there, however short.