I have noticed that one of the big problems many people seem to have with growing Gardenia as a houseplant is that it likes a steady amount of moisture but also seems to be sensitive to root rot if overwatered.
My experience has been that plants with this sort of requirement often do well in hydroculture since hydroculture allows the roots access to ample water while also having a very well-draining medium that reduces the risk of rot.
For those who have never heard of hydroculture, here is an example to show you how it works though I don't do things exactly like that tutorial. For example:
1. Instead of using a water gauge, I pot my hydroculture plants in clear plastic containers (such as deli containers or large plastic disposable cups) so I can visually see the water level.

2. I poke a hole (with scissors or such) in the side of the container at a level that allows a good amount of water to stay in the container at all times, but that allows me to easily drain out some water each time I add new water. I think it is important to flush the pot out with fresh water to avoid excessive mineral build up.

3. My experience has been that a mature soil-grown gardenia has a hard time adapting and will pout terribly (as gardenia pout at so many things), so I suggest starting CUTTINGS in the hydroton instead of plunking a mature gardenia in hydroton. If you start with cuttings, they develop roots that can live in the hydroton right from the very start rather than trying to nurse along a mature plant that has unsuitable roots until it grows new roots.

The cutting I have seems to be doing pretty well so far for several months. It's not yet large enough to bloom, but since I know many people have had a hard time growing gardenia as a houseplant I thought some people might want to try this method.

If you do try it, I will be interested to hear your results. Good luck!