Fixing a sagging gate in 20 minutes with a turnbuckle cable kit

Gates sag because the hinge-side post leans or the frame racks into a parallelogram. Check the post first: if it moves, that is the real fix (dig, plumb, concrete, 1/3 of the post below grade). If the post is solid, a $15 anti-sag kit: a cable from the top hinge corner diagonally to the bottom latch corner, with a turnbuckle in the middle. Tighten until the latch lines up. Wrong diagonal does nothing, so: top of the hinge side to bottom of the latch side.