I always wanted to do a rolling electric gate. I never got around to it. I have two 8' privacy fence panels that I bucked for gates, with turnbuckles attached to the top posts. I also have wheels to attach to the bottom, but since the turnbuckles have worked, so far for 3 years now, the wheels are on a shelf in my shed.
I suppose if one was to use a lightweight, solid wood gate of some sort and set it 6" off the ground it might not sag. Bad enough to be a problem anyway.
Posts? Buy 10'ers and sink them 4' into the ground. Bell the bottom of the hole out like an elephant's foot. Put 6" of screenings in the bottom of the hole and fill the rest of the hole with concrete. Post will not sag.
Not that I feel the need to defend my construction abilities or anything, but I was a contractor and a journeyman carpenter for the bulk of my adult life. I have no pictures of my fence ( I guess because it's just a fence), but here are some piers I dug, by hand, in the above prescribed manner. This is 10" Sonotube so the back two piers extend around 40" out of the ground. It took 90 some odd bags of concrete, mixed lovingly by me in a wheel barrow and they now support a 20X12 building which is packed fairly tight with table saws, welders, air compressors, too many tools to count, bins and bins of house stuff that needed storing and one tiny little section I call my own with a workbench I built that you could put a Buick on. Complete with hidden drawers and a discretely hinged top deck.
You'll notice I moved the doors from the gable end to the side and built a small landing. This landing will tie in with the extensive deck I'm building off the back of my house. If I can ever save up enough money for the lumber. Which is ridiculous.