Here's a quick and inexpensive fix that will have you loving your gate in no time!
Purchase an anti-sag kit at the hardware store. Stanley makes them for about $12.00.
Follow the instructions to attach the hardware to the gate. Run the cable from the bottom of the latch side to the top of the hinge side, then tighten the cable with the provided turnbuckle to eliminate the sag. Your back in business. : )