As long as your gateposts are solid, in deep enough too, it shouldn't be a problem, especially if you only use thinnish ply although exterior would be better - but heavier. We have a 15 foot gate done just like that on the front of a haybay to make a box but it is fitted to the girders; your gateposts will have to be very sound and strong. It is heavy to lift on or off but swings well although you have to watch for the wind! If it gets behind it, it could knock you over!