Water rooted gardenia
Our Extension Texas A&M, doesn't have a publication on gardenias, so we found a nice publication by Florida. For some reason, their site does not allow me to paste the url into this email. Info is listed at the end for your to search and find it. If you can't find it, please reach me directly at firstname.lastname@example.org
Other than anecdotal information, ie. gardener's blog, etc, I've found nothing that is research-based on potting up water rooted cuttings. The predominant comment was, that water rooted cuttings have very sensitive and tender roots that are easily damaged when handled. So, we would advise you to use a high percentage peat moss media and pot up your cutting in a container a good bit larger than the size of the roots. Place the pot in a shaded area and keep evenly moist. You may already be aware of this, but In the industry, woody cuttings are kept under an intermittent mist system to maintain humidity. It is closely monitored environment.