Select a level location to install the gate arch. Measure out a 3-by-4-foot rectangle. Mark the rectangle on the ground using spray paint. This will be your guide for building the arch. The short sides of the rectangle will be the sides of the arch, and the long sides will be the gate opening.
Dig a 1-foot deep hole in each corner of the rectangle, using a post hole digger. The holes should be about 5 inches in diameter. Shovel a small amount of gravel into the bottom of each hole to help with water drainage.
Cut four pieces of 3-by-3-inch lumber to measure 7-feet long, using a circular saw. These are the posts for the arch. Place each post upright in the corner post holes. Pack the dirt from the holes back in around the posts using a hand tamp.
Cut 12 1-by-1-inch boards to measure 3 feet long, using the circular saw. These are the cross pieces for the arch. Place a cross piece across two of the posts on one side of the rectangle. Slide the cross piece down so it is about 1 inch from the ground. Screw it in place with one 2-inch wood screw on each end. Attach five more across the same two posts spacing them about 12 inches apart. Repeat on the other side.
Draw a 4-foot diameter circle on a 3/4-inch piece of plywood, using a carpenter's pencil. Erase half the circle and use the remaining half as a guide to draw a 4-inch-wide arch. Trace this arch to create an identical second arch. These arches will form the top of the archway.
Place one of the plywood arches across the top of the front two posts. Screw it in place with two 2-inch wood screws on each side. Repeat across the back of the arch.
Cut four more lengths of 1-by-1-inch lumber to 3 feet long, using the circular saw. Place these four across the two plywood arches evenly spaced. Screw them in place with one 2-inch wood screw on the end of each board. This is the completed arch.
Cut two lengths of 3-by-3-inch lumber to measure 40 inches long. These pieces are the top and bottom of the gate.
Cut two side pieces for the gate from 3-by-3-inch lumber to measure 36 inches long. Arrange the top, bottom and two sides to form a rectangle with the sides coming over the ends of the top and bottom pieces. Use one 2-inch wood screw at each corner to secure the frame together. Check the angles with a right angle to be sure they are straight.
Cut 10 lengths of 1-by-1-inch lumber to measure 36 inches long. Arrange these boards across the frame of the gate 3 inches apart. Screw these boards to the frame using one 2-inch wood screw on the end of each board.
Attach the gate to the arbor opening using the gate hardware kit. Follow the package instructions carefully. Be sure the gate has plenty of room to swing.
Apply a coat of wood stain to the archway and gate using a paintbrush. This will protect the wood from weather and increase its lifespan.