Sagging gates can occur for any number of reasons, from shrunken wood and loose screws, to leaning posts and old hinges. However, you can fix a sagging gate in a day without much trouble if you take your time to make repairs as necessary. Once you've made some improvements and replaced parts which need to be removed, your gate shouldn't sag and will work again without being a problem.
Check that your gate post is level running straight up and down or close enough to level that you're certain the sagging is indeed the gate. If the post is leaning, then you'll want to fix the post before working on the gate.
Unscrew the screws from the bottom hinge and set the hinge aside. Remove the top hinge screws and set the hinge aside. Remove the gate and lay it on the ground with the screw holes facing up.
Apply a thick amount of wood putty to fill each screw hole along the gate and gate post using a putty knife. Allow the putty to harden for four to six hours, or longer if your brand instructions indicate that.
Remount the gate to the gate post using new screws starting with the top hinge followed by the bottom hinge. If the existing hinges appear rusted or the holes are worn, replace the old hinges with new ones of the same size.
Slip a wood block or shims under the latch side of the gate to prop it up and relieve tension on the hinges. Check that the gate is level and add more shims or use a larger block as needed until the gate runs level.
Install a turnbuckle tension rod to the back of the gate with the rod running diagonally. The upper end of the diagonal should be attached to the top of the gate's latch side with the lower portion of the diagonal attached to the bottom portion of the gate as close to the bottom hinge as possible.
Tighten the turnbuckle tension rod as instructed in your kit's packaging until the rod is tight, not drooping. Continue to tighten the rod until you can remove the wood block or shims and the gate remains in place without dropping downward.
Fix the leaning gate post by following steps 6 and 7 after propping up the post with rocks to make it upright. Attach the upper diagonal portion of the turnbuckle tension rod to the gate post and the lower diagonal end to the adjacent fence post.