Cut the seed pod from the gardenia when it has developed a dark skin that is rough and almost leathery and is no longer green in any areas. You will have to look diligently since only moths or butterflies tend to pollinate this type of gardenia and they are only rarely successful.
Peel open the pod with the tip of a sharp knife to reveal the tiny seeds inside. They should be brown and interspersed between the fleshy matter of the seed pod.
Separate the seeds from the fleshy matter, using your fingers or a pair of tweezers, and collect seeds on a clean dry plate. Dry the seeds for a few days so that their outer skin toughens up and becomes hard.
Store the seeds in a dry paper envelope marked with the date and the name of the seed. Plant as soon as possible.