Leaf drop on a gardenia may be due to lack of soil moisture for a period of time. Strangely enough soggy wet conditions can also damage roots and lead to loss of foliage. Lack of nutrients may cause a loss of green color especially in the older leaves but not usually leaf drop. I'd suggest you focus on soil moisture and hopefully the plant will turn around soon and stop dropping leaves. I also should add that loss of older foliage periodically is not unusual for a gardenia.
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