There are many reasons gardenia leaves turn brown, including being kept in the wrong climate, not getting enough light, a lack of humidity, pests and fungi. If the brown color is all over the leaf, then it usually means that the leaf is starting to die, whereas a few brown spots signify fungi growth.
To avoid brown gardenia leaves, keep the gardenia plant in its optimal climate between the temperatures of 68 degrees Fahrenheit to 74 degrees Fahrenheit. The gardenia should also be kept out in the sun whether it is indoors or outdoors and should be kept in a high humidity environment. Any pests should be kept away, such as spider mites or aphids, and if a fungi is present then use a fungicide treatment.