Sag is the gradual drop of a swing gate's leading edge, measured from where it was when the gate was new. After a few months or years of use, the leading edge can be 5 to 30 mm lower than it started, depending on gate weight, hinge quality, and how plumb the hinge stile actually is.
The causes are cumulative. Gate dead weight torques the hinges, slowly stretching them. Wind load over many storms adds dynamic load that the static design didn't account for. Any out-of-plumb in the hinge stile gets worse over time as the gate's weight settles the post slightly.
Fixes: a drop bolt at the leading edge, dropped into a ground sleeve when closed, supports the gate weight directly to the ground. A turnbuckle in a diagonal cable across the gate face can be tightened to pull the leading edge back up. Worst case, replacement of the hinges with longer or heavier units is the cure.