Certified GKH Gardening Expert
Typically, you want to plant it flush with where it was planted before. In this case, you can plant a few inches deeper to help support it, but do not go any deeper than that.
The root ball will extend out about 6" beyond where the canopy of the plant extends. You will want to go down as deep as possible. This article will help as well: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/transplanting-gardenia-plants-planting-gardenia-somewhere-new.htm