Kelpie,
Gardenias like to be snug in a pot. They are very prone to root rot, so the more appropriate the size for the small plant, the less chance of root rot.
When they start rotting, you will see brown leaves, and they start towards the bottom of the plant and work their way up.
Is there a reason you didn't plant it in the ground?
I would buy a pot (regular pot, clay) just big enuf for the plant. Repot it, and let it dry out some.
When you are buying your new clay pot, pick up a moisture meter, it is a godsend with gardenias.
Only water the plant when the moisture meter says dry when you place it down near the roots to check.
It should be fine if you do this.
Good Luck.