most slotted posts have a hole through them so is it possible to use them to attach a 4x2 onto the post and fix your hinges or latch onto the edge of it?
like this with the hinge or latch fixed to the short edge of the timber:
Another option is fix 2 on like that (one either side of the post) and then a 8 x 2 across.
like this:
Depending on it being the end of the run or not may mean resizing one panel or two or none of you are lucky but its not difficult with either lap fencing or feather edge.
Lap is the more difficult but just remember to move the edge beading's in to their new positions and nail from both sides making sure all the laps are held firm then cut the ends sticking out off carefully so you do not split them.
If you are concerned about the cut looking poor cut the other end if it is going into a slotted post as it will hide any errors.
Then refix the panels onto the face of the timber with fencing brackets.
Oh and do not put the timber all the way to the ground as it will rot at the bottom real fast if you do.
And while it sounds odd concrete posts will bend a bit under the weight of a gate so account for that when you hang it.
For a heavy gate i would fit a metal post in concrete down the side of the concrete post.
