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This can be due to a large number of things. If these are newer leaves, then it can mean that you have a nitrogen deficiency. If it is the lower leaves it could indicate that There could be a lack of light, over-watering, under-watering, magnesium, calcium deficiencies. Really a great number of things.

Most commonly, a simple pH adjustment will correct most of these. You can add dolomitic lime to keep the pH stable, but it would be best to get a cheap test kit and follow the instructions to find the exact pH. Adjust accordingly to keep it around 6.5. Iron sulfate will help lower the pH if it is too high.

Here is an article that will help: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm

How often do you fertilize this? If the answer is rarely, then I would say that this is the issue. If you don't want to fertilize then you can just repot it with brand new soil.

Here is an article with more information: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/fertilizing-gardenias-in-your-garden.htm

If you mean gardenia, check the following articles for possible diseases. Sooty mold is a possibility:
http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2058.html
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm

Otherwise, you might be right that it was planted a little late, but there's not much you can do now. You will have to wait for spring and hopefully it will get better established then. It may also just be a symptom of cold weather exposure, which the plant should recover from in the spring.

This is a good guide that will take you through the process: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm

Gardenia are known for their finicky ways!

Here is a link to help you determine the cause of black spotting.

https://www.gardeningknowhow.com/ornamental/flowers/gardenia/brown-spots-gardenia-buds.htm

It is likely that they will survive on their own. When you start to see new growth, just prune off anything dead. This will help it to recover. Just keep an eye on them, and they should survive. If they start showing signs of infection, then you will want to put wettable sulfur into the soil. This will help keep away the infection until they can recover.

Here is an article that will help: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/transplanting-gardenia-plants-planting-gardenia-somewhere-new.htm