It's safe to keep gardenias if you own a cat, as they are generally considered relatively safe for cats. Gardenia flowers are typically considered non-toxic and will not cause any direct health problems for cats.
However, caution is still needed when allowing your cat to come into contact with gardenias, and you should observe their reaction closely. Some cats may have allergic reactions to the plant or flower, which may manifest as vomiting, itchy skin, or indigestion. If you notice any unusual symptoms in your cat after contact with gardenias, it is recommended to consult a veterinarian immediately for professional advice.

Additionally, it's best to ensure the gardenias haven't been treated with toxic chemicals. Take care to prevent your cat from accidentally ingesting or chewing other parts of the plant, such as the leaves or stems. If you notice your cat showing excessive interest in gardenias, consider placing the flowers where they are out of their easy reach.