I have only had experience with Gardenias indoors since they won't survive our winters outside. However it has been my experience that they like to be kept evenly moist, bright light, and require special fertilizer. Yellowing leaves is usually overcome by using a fertilizer which is formulated for acid loving
plants, try using a product put out by the makers of Miracle Gro, it is called Miracid. The NPK should be 30-10-10. Gardenias are also very susceptible to spider mites so you might want to mist the
plant frequently (avoid getting any water on the blooms). Good luck!