A fence gate needs to be lifted when the bottom edge of the latch side of the gate hangs lower than the hinge side of the gate. Long heavy fence gates are usually the most problematic when it comes to this issue because of the distance from the hinge end to the latch end. These gates, if swung to hard, can cause the hinges to shift thus causing the latch end of the gate to hang lower than the hinge side. The fence post upon which the gate hangs can also lean inward causing the latch end of a gate to hit the ground. These gates have to be held up by hand to close. Lifting a gate might require a days worth of work, but it can save the headache.
Remove the gate from the hinges on the fence posts. There are usually two-four hinges holding a gate up. The way the gate is removed depends on the design of the hinges, but most gates can be simply lifted off the hinges. Inspect the hinges and replace any that are bent or damaged.
Inspect the level of the fence post by placing a level on the top of the post. If the leveling bubble isn't centered the post will need to be removed and repositioned. Dig the post out of the ground using a shovel. When the post can spin freely in the hole spin the post so that the side that the hinges are on is opposite of the gate opening. Remove the hinges and reposition them on the unhinged side of the post. This will give new support strength to the hinges.
Position the post in the hole with the level on top. When the leveling bubble is centered begin placing soil in around the post with a shovel. Tap the soil down with the handle end of the shovel to pack the soil tight around the post. This packing will keep the post in position until the hole is refilled. Use concrete for fence posts upon which large steel gates are hung.
Rehang the gate on the hinges and allow the gate to swing open and closed a couple times. Watch to see that the gate has been lifted properly. The height of the gate, from the ground, should be equal on both the latch side and the hinge side. If the gate has not been lifted or the weight of the gate causes the post to shift again concrete should be used to anchor the post upon which the gate swings.