First, the gate frame and installation – it has to be measured on the site, and planned in accordance, not the other way around. If you offered a gate that was designed in the office, and suggested to adjust it while installing – you may find yourself in major issues real soon. So make sure to have your gate structure designed by licensed and experienced firm, with gates their main business.
The second important factor is the frame type. Some gate builders build the gate from wood frame, or worse, thin metal frame. When you do that, the gate is almost guaranteed to sink, bend, and loose its shape in short time. The right way, which will always cost more, is to have a massive metal frame, and wrap it with the wood planks.
The third factor is the materials and the quality of the whole finishing. When using high end, clean cut, quality wood planks, you want to make sure they stay shiny for the years to come. Oil or varnish is great way to do so, but you got to choose the best materials, and carefully apply it.