Gravel can be quite attractive to cats -- after all, it does look a lot like cat litter. Unless you're willing to get rid of the giant litter box, your other option is to convince the cats gravel is not as great as it seems.
Spray your gravel with something that cats will find as revolting as possible. The bad news here is that not all cats dislike the same smells, so you might need to try a few things before you find what works. Citronella or lavender essential oil, orange or other citrus oils, cayenne pepper, vinegar and alcohol are all potential deterrents. Choose one you can live with, especially if you have a large area to spray -- the smell can get overwhelming and you don't want your whole garden to smell like a giant salad.
Make the gravel hard to access. If you only have gravel in a small section of the property, surround it with thorny bushes or netting. Most cats will move on to another area if they have to work too hard to get to the gravel. Add some citronella plants around the area to make it even less attractive.
Sprinkle broken pine cones or pine straw on the gravel. Note that both will change the look of your gravel. If you don't mind the new texture and color, this might work as a deterrent, as it will feel uncomfortable on cats' paws and they'll be less likely to walk -- or dig -- on the gravel because of it.
- If it's your own cat using the gravel as a litter box, you have two options. If the gravel is outside, the solution is simple: keep Kitty inside. He shouldn't be roaming the outdoors anyway, for his own safety. If the gravel is inside the house, it's probably on potted plants or some other small area. Either block access to those areas -- close the door -- or move the plants somewhere where Kitty can't get to them.
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