In my opinion, fixing poorly designed wood fence gates (no diagonal cross-brace) with those cable and turnbuckle kits just delays the problem slightly. It just slowly bows the gate, if it doesn’t have a lot of rigidity to start with. Jack it up somehow until it’s square and install a 2×4 diagonally across it, screwed with good screws into as much good wood as you can.
As an aside, I think it’s marginally better with a weak worn-out gate to make the diagonal cross-piece from the high corner of the hinge side to the low corner of the free side, but it really depends on the condition of the gate.