Making The Bias Cut
added on top of the posts as connectors. Cut a piece of 1 5/8" O.D. matching piece and slide it into the top. The length of that piece should be 1' long to add a foot and 2' to add two feet to the height.
The end, corner and gate posts are a little trickier because there are no sleeves to fit that size. You must find a piece of pipe that will fit fairly snug on the interior of the post. Cut it about 2' long and use it as an inside sleeve. Cut the 1-2' extension to match the O.D. of the posts to extend and slide the extension over the sleeve. You can attach the extension in advance to the sleeve by drilling a hole and using a screw to secure the extension to the sleeve. We weld them on. The sleeve needs to extend into each side a minimum of 12".
You hit the problem right squarely on the head. It's called a diagonal truss.
(fig 1) Most hi-tensile light gauge wire will work. Hi-tensile simply means it can stretch a long way before it snaps. A lot of hardware stores sell what is commonly called a "non-sag gate kit" or something similar. We sell them also. Those typically use a wound, stranded cable instead of a wire. This type is used for wood gates normally. The hardware that comes with the kit is set up for wood. Replace the lag bolts with self tap screws or weld the steel plate fastener onto a steel gate.
On heavy or large gates, we use a 3/8" diameter galvanized steel rod, threaded about 6-12" on the end. The plain end is welded or bent to loop though the top hinge side of the gate and the other end is passed through a hole in a turnbuckle that is nothing more than a "L" shaped steel piece fastened to the low sagging corner. A nut is placed on the threaded rod. As it is tightened, the gate rises.
(fig 2) On very large gates (over 18'), several trusses are placed, if needed. The first starts at the top, hinge side and secures at the bottom of the first vertical brace. The second fastens at the top of that vertical brace and goes to the bottom of the next vertical brace. And so it goes until each section of the gate has it's own truss. Each truss can be tightened individually so the gate does not have a chance to sag in the center. Only one truss would lift the end, but the center could sag.
It could be that the band bolt holes are incorrectly made, but if the problem persists with all of the bands, you are probably not installing the band properly. It is best to install the band by sliding the band down over the post to avoid bending it out of shape when you force it around the post by pushing it on from the side of the post. Some bands, like heavy flat or the beveled type are very rigid and the act of pushing it onto the post from the side is worse than with a light-weight flat type band.