Building a gate is a difficult job for most people. I recommend that you use a steel gate _frame_ kit. I makes gate building easy and sure. Some gate _frame_ kits come with all the hardware and some do not. Most gate kits will utilize a cable in tension top hinge side corner to the bottom of the opposite side to help keep the gate from sagging. This will help for a short time, but the real answer is to purchase a steel _frame_ gate kit that utilizes solid bracing in both compression and tension (corner to corner in both directions. If you need more information about this please let me know.