How to attach a bolt down post support to a fast set concrete post built with

Assuming that they are round metal poles that are generally used for chainlink, the 'corner' poles are what you generally use for a gate. You pretty much use corner poles anytime you terminate a run of any kind, and a gate opening is a termination. The actual gauge isn't going to matter so much as the diameter of the pole, you need to make sure your gate hardware will fit the pole. Since your are going with wood, the extra few bucks for the thicker gauge is probably worth it, the actual diameter should be about the same either way.

Also, make sure you measure the opening correctly before concreting the poles in. Depending on the hardware you use you may need to leave extra space. For pre-fabricated chainlink gates, you need something like 39 inches for a 36 inch gate. Since you are using wood, I assume you are fabricating it yourself, but you'll still want to make sure you leave plenty of room.

I recently saw someone set a length of galvanized pipe in a concrete footer, and then bolt a pressure-treated fence post to the metal pipe. The galvanized pipe and footer should last essentially forever, and provide plenty of strength to support the fence/gate. The fence and gate can be replaced as needed. I thought it was a fairly clever approach. You might consider it for your application.

It's perfectly normal to have two gates without a center post, using a ground pin (often called a cane bolt) to hold the less-often used side of the gate stationary. The only downside here is that the gate will twist a bit more if side loads are applied near the top of the gate (e.g. wind, someone pushing on the gate), since the only fixed point is at the ground.

The longer the cane bolt, and the further up the gate it extends, the more support it can provide to the gate. Taking this to the extreme, if you were really concerned about the rigidity of the swinging end of the gate, you could make your own "cane bolt" out of a long section of pipe (perhaps 4 feet), and have it drop into a larger pipe set a foot or so into the concrete. That gate wouldn't go anywhere!