How to attach a bolt down post support to a fast set concrete post built with
If you haven't poured concrete yet, use "anchor bolts". (Ref: Simpson Strong-Tie Model # SSTB16-R, for a visual, though it's probably too long and wrong diameter.) I'd use ones around 12-16" long and as big a diameter as will fit. Get galvanized. Bolt them into the base (nut on top and bottom) and use that to maintain alignment while the concrete sets. Once the concrete has set, remove all the nuts, set the base, shim as necessary with metal washers, and bolt it down. For bonus points, add concrete below the base after shimming but before final bolt down. 

I can't remember if you have a section of fence adjacent to the gate, but do your best to tie everything together as much as possible. Gates are hard on posts.