Gardenia not healthy looking
Gardenias are a bit finicky to grow as houseplants. They like more humidity than what is usually available indoors. It's good that you're growing your plant in a sunny window, because they also need bright light indoors.
General yellowing of leaves may be due to soil pH that is too high. Gardenias grow best with acidic soil. Most water sources tend to be alkaline, so over time, the soil can become too alkaline with normal watering. Try using an azalea & rhododendron or a citrus fertilizer instead of an all-purpose houseplant fertilizer. These two types of fertilizer are usually acidic, and will help keep your gardenia a healthier green. You may also need to apply iron chelate to the plant, especially if the yellowing pattern on the leaves shows up with green veins running through the yellow areas.
The occasional black spots you see on the leaves are not likely a serious problem. Old leaves naturally turn yellow and drop off, and sometimes will develop black spots, too.
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