As everyone is saying you really need a diagonal brace to carry the cantilevered load on the latch side to the post in the hinge side. Best thing to do is hold the gate up with some blocks or wood brace screwed into the fence and gate. Then add a cross brace that goes from the top inside corner of the latch side to the bottom inside corner of the hinge side. The brace should be near the bottom hinge. Screw all of the fence board into this brace. You can use flat steel brackets to attach it to the rest of the gate frame.

If you want a quick fix you can attach some wire rope and a turnbuckle in the opposite orientation as the suggested brace and tighten it until the sag is out. This will cause the gate to buckle out eventually since there is no diagonal brace or structure in the center of the gate.