After too many years of fixing wood gates I'm pretty opinionated
First, with gates that heavy a 4X4 post is marginal and will need to be well into the ground. 3' is my minimum and I prefer deeper when helper is doing the digging
. I'd use a 6X6 there if it were my fence. I also like to compact the soil around the post after the concrete sets up with a heavy 4X4 or my sledgehammer head used like a tamp, mounding the top slightly for drainage. Second, I want a one piece diagonal brace running from hinge-side bottom to center top. I want that brace to fasten to the vertical stiles, not to just push on the top and bottom rails. I like "T" or "strap" hinges placed allow at least one hinge fastener to go into that brace an the bottom. Third, I want all four corners of the rectangular framing screwed together with 6" screws which have large flat-bottomed heads. Top and bottom rails cap the side stiles. Fourth, all hinges and hardware must bite at least 1 1/2" into the framing; supplied screws are almost always too short. Fifth, either use hand-selected straight-grain treated wood or the premium pre-dried treated wood for framing as the usual stuff warps and twists in time. If you want overkill, use construction adhesive at the joints and also where the fence slats attach; enough to hold but not 'squish' out
Gates and their posts are the first point of major failure in wood fencing; they get the most wear and tear. There is a lot of leverage on the post when they're open(especially with a kid swinging on them
)My gates outlast all the rest, usually by a factor of 2-3 times. Usually by the time my gates are going away the rest of the fence is too. Those store-bought gates will be falling apart long before they need to because they are built to an economic point, not a quality point. If my customer insists I use them I make sure they understand that I am not warrantying them in any way at all and I put that in writing with their signature agreeing to those terms. Those folks almost always call me back in a few years to build them a set of gates and then they don't complain about my price because they now understand it's worth it.
Phil