An easy way to cover gabions is to allow creepers to grow up them from the bottom. To do this, all you need to do is to plant climbing plants such as ivy, clematis, Virginia creeper or even roses in front of the gabions. This method is particularly good for high gabion walls as the plants use the wire mesh to climb.
If you like simple but full greenery, you can plant turf inside the gabions and fill them with soil. The top will also be covered with grass resulting in easy to maintain walls or cubes.