Seems no matter how deep or how big I make my gate posts over the years and wet seasons they eventually want to sag. To resolve this problem when I set a new or replace an old gate I drive a short piece of 4x4 at the base of the post on the non-hinged side of the gate. I get the gate level and then drive a 4x4 that I cut a wedge point on one end down beside the latch post one inch above the bottom of the gate. When I close the gate I just lift that one inch up and set the gate end on top of the 4x4. It keeps the gate from pulling on the hinge side post. I use some 16 foot gates and even if the hinge side post don't sag the gate will. This stops all the sag.