Preface *I'm no engineer*
I would imagine you would need a shit ton of concrete to make for certain it never starts to sag while holding all the weight. Does the gate stay open or closed most of the time? If closed, I would do a guy wire from the top back to the bottom of a second post with a solid bar spanning the tops, like a high tension electric fence corner. Or just get one of those wheels on a spring to take some of the weight off 
