My gardenia is a houseplant in the winter. It has some sort of leaf disease, which doesn't seem to fit the descriptions I have seen. Some of the leaves (old and young) begin by turning yellow up the middle and gradually turning all yellow before drying up and falling off. They also have a couple of spots, about 1/4 inch and green with a solid black center.
Gardenia Leaf Disease
Certified GKH Gardening Expert
The yellowing sounds like it may be an iron deficiency. This article will help with that: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm
The spots sound like an unrelated fungal issue. It probably started because the plant was weakened by the nutrient issue, but needs to be treated separately. I would recommend using neem oil. It will treat the fungus with the added bonus of keeping pests away, which gardenias indoors are also prone to. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm