To tar a garden gate, treat it like exposed outdoor timber: clean it, dry it, brush tar into the vulnerable edges and joints, then allow it to settle before regular use.
A garden gate is a good tar project because it has all the weak points of outdoor wood in one small object. The top edge catches rain, the lower rail is splashed from the ground, hinges create exposed holes, and moving joints can hold damp. Tar helps by darkening the gate, slowing water entry, and giving the surface a coating that can be renewed.
Remove loose dirt and old failing coatings first. Brush tar onto the broad faces, then spend more time on end grain, the tops of stiles, the underside of rails, and any exposed screw or hinge areas. Keep the coat thin around the latch and hinge movement, because thick tar in those areas can mark hands, clothing, and nearby posts.
A tarred gate may smell strongly at first and can remain tacky if the coat is too heavy or the weather is cold. Leave it open or partly open only if it can be supported safely and kept from rubbing against pale walls, paving, or plants. When the finish dulls and water stops shedding cleanly, a light maintenance coat is usually enough.