Certified GKH Gardening Expert
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