Gardenia are a tropical plant so the temperature needs to stay above 40F day & night. They also need good light so the heated greenhouse would be best or a room with good life. by keeping in small containers lightly water & no fertilization they can be kept for longer periods of time.

I'll offer a friendly disagreement to yiorges temperature comment. I think that Gardenias may qualify as a semi-tropical plant, rather than tropical. Even the non-hardy cultivars can withstand colder temps than 40F. Hopefully, you took your cuttings from one of the many cold hardy hybrids which will give you a great temperature range.

But you will still need to protect your cuttings while so young and still in containers. Bring them inside to that cool room during times of sub-freezing weather, especially if your greenhouse heats up like crazy during the day time. Nothing worse than an 80F day (inside the greenhouse) followed by a 30F night!

Do you know which Gardenia it is? Only the hardy ones will survive in zone 7.

Just saw a post in another forum from a gardener in zone 6b who has been successfully growing gardenias in the ground for 7 or 8 years. FYI, bama!