I can't see the silver thing from the other side of the fence.

The silver thing is a nut. If I remove it, I have the "tail" of a
bolt. This bolt is coming out of the block wall & through the hollow
post. I can only assume that it was pushed out when they built the
wall. Also removing that nut will free the post from the top of the
wall. I think I explained it pretty well, except I wasn't clear about
it Stopping the gate from moving the last inch.
I made it sound like it was binding or friction. That was last year,
now we've moved on to the gate slamming into the nut. Anyway, I'm
probably going to do Smitty's solution, and install a flat/flush head
in another hole, and cut the problem bolt out of the way. If it goes
smoothly, I might also do the bottom bolt/nut. That one has plenty of
clearance for now.