A while back I was doing some painting in the back yard of a customer's house. Assigned to patrol that back yard was a pit bull named Moses. I would peek over the gate and whistle and if Moses was there he would come barreling around the corner.
Well, one time ol' Moses decided to play possum. I whistled and no Moses. Thinking he was locked up inside the house, I confidently opened the gate and grabbed the paint. As I turned the corner I saw Moses on the other side of the pool. Then I realized how he got his name. I swear Moses ran across the pool to come get me. I made it through the gate and the mesh prevented his saliva from drenching the back of my shoes.
You can buy pet screening or mesh at a home center for less than $20 and you can choose from a few different colors. If you have an extra large gate, you may have to buy a few pieces. This mesh will also offer some privacy from the street, as the holes are quite small.
Be careful with this stuff because the edges are sharp. Use tin snips to cut the mesh. After you cut the sheet to fit your gate, the edges are downright dangerous.
You have a choice either to precut the mesh and screw it on the gate, or screw it on the gate first and then cut around the gate. If you choose to cut around the gate, as you cut, the mesh will have the tendency to curl, which may cut you or Fido.
Either way, you may want to wear heavy leather gloves when you handle it. A must for this job is a drill driver fitted with a Phillips head bit.
You need self-tapping, pancake-head, galvanized screws, which eliminate the need to predrill a pilot hole into the gate. Self-tapping screws cut a hole as they hold the surfaces together. Some mesh comes roughly 36 inches wide by 72 inches in length, so you will need to cut it before you hold it up to the gate.
Use clamps to hold the mesh to the top of the gate, but cover the jaws with wood scraps or something that won't scratch the paint. Once you have it in position, put a screw in the drill and hold it up to one of the holes in the mesh. Choose either the upper left or upper right corner of the gate and not too close to the edge.
Start slowly until the screw starts to cut into the gate and stop when the screw head holds the mesh snug to the gate. Align the rest of the mesh to cover the gate and add screws about every 8 inches along the top of the gate. Remove the clamps and continue down the gate making the bottom the last section to screw together.
If the interior bars of the gate allow, screw the mesh to them as well, otherwise every time the gate closes the mesh will bang against these bars.
If the gap between the sides of the gate and the wall are so large that Fido can fit through, you can screw a piece of angle iron to the gate, effectively widening the gate. This goes for the bottom of the gate as well. Just spray some paint on these parts to protect them from the weather.