H-brace. 2 posts with shorter post nailed in between to make an H shape. Six foot post is a guarantee for failure. For gates that heavy, you need to sink the posts DEEP. If you want four feet above ground, then sink your posts four feet in the ground. If you only sink posts a couple of feet, they will heave and lean. More feet of post underground than above is even better.
Under the H bar, nail another brace, starting just under the h brace on the outside post, slanting to ground level on the inside post. The ends will have to be cut at an angle to fit against the flat posts. This will help distribute the weight and help keep the gates from pulling the posts out of whack.
You can also make your inside post 3 or 4 feet taller than the gate and use a guy wire. This is a support wire run from the end of the gate to the top of the post, secured on both ends. I've done this, it helps keep gate from sagging.
Now, have I made this all clear as mud?
They are much heavier then I thought at least 300 pounds each. I was going to have 6 by 6s cemented into holes. Then bolt hinges on, nephew thinks weight of gates over time will pull out bolts. What is your suggestion to hang them ?
