Certified GKH Gardening Expert
It could, absolutely, be overwatering. You should only water when the top two inches of soil are completely dry.
It can be safe to use a light fungicide, but if you are in doubt, then there is a safer alternative.
Wettable sulfur will kill of most infections in soil, but can be a little acidic for some plants. So it should be applied with dolomitic lime. This will keep the pH of the soil stable, as well as provide calcium and magnesium back to the soil, while the sulfur kills the infection.
This article will give you information on the general care of these shrubs: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm