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.
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.