Java fern has no feeder roots, so no, it won't grow simply by planting it in gravel, nor will it attach to a smooth surface, including slate unless it has a lot of small faults for the fern to grow into, so the most common way used to attach is by tieing it, usually with fishing line, at least long enough for it to grow into tiny faults. Then, you can cut away the line before it stretches and becomes a fish trap.