Access is one of the key factors to how successfully your fence serves you. Having gates that work well where needed is essential. When you have a need for a gate in a line of fence on a slope, it presents a unique challenge, which after all is part of what do-it-your-self builders look for in a project. Remember, that just because the ground tilts, does not mean that your fence should too, you will want to plumb your posts completely upright, just as if you were working on perfectly level ground, before beginning to build your gate.
Measure the distance between the posts. Mark and cut a fence panel of the same material as your fence, 1/2-inch narrower than the narrowest part of this opening. Use a chalk line to mark the panel, and cut with a circular saw.
Drive a screw into each gate post, 3 inches above the ground and tie mason's twine between them. Stand the gate between the posts, tipped flush against the uphill post. Trace the line of the string onto the face of the gate. Draw a line parallel to the traced line, one inch above the bottom corner of the downhill side of the gate. Cut the bottom of the gate off along that line with the circular saw.
Install a gate sag kit on the inside of the gate, following the specific instructions that come with your kit. In general, fit the corner brackets to the top hinge side corner of the upper horizontal brace and the lower latch side corner, with the cable and turn buckle between them. Fasten the brackets with the provided screws. Tighten the turnbuckle until it is snug.
Place the hinges on the face of the gate, aligned in the center of the upper and lower horizontal braces. Mark the bolt holes onto the gate and bore pilot holes in each marked location with a bit equal in thickness to the hinge bolts that came with your hardware. Attach the hinges, tightening a washer and nut to the inside of each bolt with a socket wrench.
Stand the gate upright, with a 1-inch thick wood spacer under each end between the posts. Position the hinges and bore pilot holes through the horizontal rails of the panel on the downhill side of the opening. Bolt the hinges to the fence and remove the spacer. Test the swing of the gate to be sure it clears.
Install the gate half of the latch first. Bore pilot holes and drive the provided screws to secure it according to the instructions with your hardware. Mark the center of the latch bolt on the uphill post and install the post half of the latch centered on this mark.