Consider this. Instead of a "gate" it would be far, far simpler to make two of the panels removable. They're somewhat heavy but then how often are you going to be moving them? (if they're too heavy make three 5 footers.) Make the center post(s) between the panels a medium diameter pipe post. Set it into a pipe that's buried 2 feet but only sticks out of the ground maybe 3 inches. Make set-in brackets that you bolt to the wood end and weld to the center post. When you need 8 feet just lift out the most convenient 8 foot section and lean it against the other. When you need more remove both panels and the post for a full 16 feet. Gates of that size are impossible. They always end up looking terrible. If you must gate it build a metal frame around the entire gate and criss-cross it with bracing and always put a wheel on it. You'll never be able to suspend it.

Set your end posts. Stretch a line. Set the rest of the posts on the line. Set the posts taller than you need and then cut them off when you're ready to hang panels. It's hard enough to manage two dimensions without having to worry about the height at the same time. Use a transom or a laser level (any rental place) to mark the height. Whoever you get your panels from can probably point you to a book that works well with their type of fencing. cheers