Experts and BHG readers answer.
Yellow leaves on gardenia tree
I think your gardenia is drowning. The soil for a gardenia should remain moist but never waterlogged. This will cause the leaves to yellow and drop. they bloom best when they are kept between 65 and 70° during the daytime and a little cooler at night. You do not say if the plant has been kept indoors or out during the summer. Too high or too low temperatures affect blooming. Overwinter the plant indoors in bright light but not direct sun, and start feeding it with half strength acid fertilizer (such as Miracid) every 2 weeks or so when growth begins in early spring.