Another consideration is that the castor swivels around the centerpoint of the mount and the wheel is either inside or outside the centerline of the mount depending on which direction it is rotated. This changes the support point relative to the gantry's center of gravity and can make your gantry crane less stable.
In my terrible sketch below, Looking at the gantry from one end, with very exaggerated yoke of the castor on the left turned in the position shown, the center of gravity of the gantry ( the black dot at the top) is centered over the point where the left castor wheel rests on the ground. The gantry is in danger of tipping over to the left.
Obviously your selected castor will not be so extreme, but be careful because the position of the wheel does change the stability of your gantry crane.
