yellow leaves on a gardenia.. my first guess is over-watering. How often do you water? Is it in a pot? How many hrs of sunlight per day?

There are several reasons Gardenias yellow. Like Kem said, the first and main reason is over-watering.
BTW, did your Gardenia have aluminium foil covering its pot?
Most nurseries/stores that sell Gardenias, pretty them up by clothing colored foil around a simple, growing pot. All the foil does is hold water and drown roots.
Most often, the roots start rotting before a consumer brings it home.

What's the longest you've had a Gardenia? If they died fast, it's possible they weren't getting the proper care in the first place, and/or over-watered.
Don't give up. Toni