Prior to departure for a dual instructional navigation exercise, the instructor pilot obtained a weather briefing which, although indicating marginal Visual Meteorological Conditions, he considered suitable for the exercise. In the initial stages of the flight the weather was better than expected and conditions ahead appeared to present no significant problem. Later in the flight the weather deteriorated rapidly with patches of low cloud developing. As a result, the pilot decided to carry out a landing on a dirt road which appeared suitable from the air. The approach and landing were normal until, late in the landing roll, the aircraft collided with a partly open gate which the pilot had not seen prior to landing.