Qwerty,
You don't show where you are located. So weather/climate considerations are unknown.
The suggestion of a breathable cover is right on the mark. The sun will probably do more damage than rain or birds. It will kill the faces on your speedo, make the lenses cloud or yellow, will fade the paint, destroy your upholstery and make your tires age faster.
The problem with gravel is moisture is always under the bike unless you life in New Mexico or Arizona. So, if you decide to put down concrete pavers (Home Depot, Lowes and other building supply houses sell them) I would suggest that you do as follows.
Excavate the are so the pavers will be flush or just slightly raised about the gravel surface. Put a layer or two of polyethylene sheeting down and crown it in the middle so that any rain water will run to the outside. You can then put a light layer of sand over it into which you'll set your pavers. You could also use stone dust or your existing gravel. With this method, the moisture in the ground will not rise up and keep your covered bike in a constant atmosphere of very high humidity. If you have large climate swings (cold nights, warm days) the ground moisture could condense on the bike and it will never be truly dry under the cover.
If it were me, I'd buy a small shed and put it in the shed rather than leave it outside with a cover. If you do that, then put poly under the shed as well. I know of two cars, a Triumph GT6 and a DeTomaso Pantera that were both destroyed by covering them while parked on bare ground. The Pantera all but dissolved over the course of a few years. For what you will pay for a good, breathable cover, you could probably buy a cheap metal shed.
regards,
Rob