Most gravel will not repel cats. In fact, a gravel-covered flower bed looks a lot like a giant litter box to many cats. Gravel is easy for cats to dig in, allowing them to bury their waste and mark the area, but there are some other solutions.
River Rocks
Many gardening experts recommend covering the areas of dirt between your flowers and other plants with large, flat river rocks. Since cats can't dig through the rocks to get to the dirt, they will simply move on to another, easier location. The rocks have the added benefit of creating a unique landscape in your garden.
Plastic Spikes
To deter digging in the garden, you can lay spiked mats made of plastic or rubber just under the surface of your garden. The spikes won't hurt the cats and other animals that enter your flower bed, but it will annoy them. There are commercial mats on the market made specifically for this purpose, but you can also use a clear plastic rug saver, like those sold at office supply stores. Just place the mats spike side up and cover them with a thin layer of soil or stones.
Natural Deterrents
Cats dislike the smell of citrus, peppers, garlic, onions and tea. Adding any or all of these things to your flower bed should keep cats at bay. You can either make a spray from essential oils or a puree paste in the blender. Chunks of lemon and orange peels have also been reported to work as natural a deterrent when spread around a garden. Just make sure to reapply your scent every week and after every rainfall.
Physical Barriers
Prevent cats from digging in your garden by laying lattice or chicken fencing down throughout your garden. The barrier will make it too hard for cats to dig in the flower bed and cause them to look for a new, easier area to do their business. You can either plant your flowers through the holes in the lattice or cut the barriers to fit around your preexisting plants. If you don't like the look of the lattice or fencing, cover it with dirt, mulch, gravel or stones.
Electronic Devices
There are a number of motion-activated devices on the market to keep cats and other animals out of the garden. One device uses a high-frequency noise to scare cats away. The noise is imperceptible to humans, but is supposed to annoy cats. Another common device is a motion-activated sprinkler. When the sensor is tripped, the sprinkler sprays water for about 10 seconds. The water will chase away most cats.
Distraction
One theory suggests you plant a cat-friendly garden away from your main garden as a distraction for neighborhood cats. You can plant catnip and rye oats and provide soft soil or sand for the cats to dig in. If used in conjunction with any other method, a cat garden would certainly provide offending cats with an easier and friendlier option.