Help with strange type of bolt

This is a specialty bolt made up my the manufacturer of the basketball hoop. I suspect you are asking because you need a new one. I doubt if you will be able to find one, but you should be able to make up something very similar that will do the same function. Get a regular bolt, washer and nut, with the right diameter and length. A second washer might be good enough to sandwich in the the square bar to the frame. If not, get a small plate and drill a hole in it for the bolt. The plate will serve the same purpose as the "leg" on the existing bolt.

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.