A gate is only as good as the posts it hangs from and latches onto. When gateposts are sturdy, they will stand up to the swing and slam of each opening and closing. But when they're loose in their holes or are weakened by rot or insects, then hinges turn creaky, the latch won't catch, and the gate can’t do its job.
A post that's wobbly but still solid can be fixed quite easily by packing crushed stone around its base with a sledgehammer. But when a wood post is rotten, it has to be removed and replaced.
A sagging wooden gate is a nuisance at best, and can become impossible to open. The problem is pretty common, and gate repair kits/turnbuckle kits are readily available at home improvement stores.
The kit contains two metal corner brackets with mounting nails, two lengths of wire cable with galvanized metal U-bolts (which are used to attach one end of each cable to one of the corner brackets and then the other end of each cable to the turnbuckle), and a galvanized metal turnbuckle.
To repair the gate using a kit, attach a metal bracket to the upper corner of the gate (on the hinge side). Then mount another bracket diagonally at the lower corner of the gate on the latch side. Attach cables to each corner bracket and then to the two ends of a turnbuckle.
As you tighten the turnbuckle, the latch side of the gate rises. As you loosen the turnbuckle, the latch side of the gate should drop. Keep in mind that this system won’t work if the upper bracket isn’t placed on the hinge side of the gate.
Here’s how it should look when you’re finished…