Pale gardenia leaves invite an immediate diagnosis of iron shortage, but appearance alone does not tell you why the plant is struggling. Start with the growing medium, water, drainage and the pattern of affected leaves.
Write down what the plant was potted into and whether it has recently changed position. A purchased plant may still have its original root ball inside a newer layer of compost.
Look for a pattern
Are new leaves paler than older ones? Is the whole plant losing colour, or are a few old leaves changing? Are veins still more visible than the tissue around them? Photograph those details in natural light before the leaves fall.
Check the root environment and use reliable advice for the identified gardenia. Do not add several acidifying products and fertilisers together in an attempt to cover every possibility.
A test of the growing medium or local water can be more informative than guessing from a picture. If symptoms continue, bring the planting and feeding history with your photographs when seeking advice. The condition of the roots and medium belongs in the diagnosis alongside leaf colour.
Related reading: Small-Leaf Gardenia Care: Soil, Light and Watering.