Congrats on getting Gardenia cuttings to root.
What are the length of roots and how long will you keep as they are until potting in a different container??

You said you tried a variety of setups. Were the cuttings in straight Perlite? No other medium?

About keeping in your garage during winter.
Since you're in z5B, it probably gets pretty cold. What's your garage temps in the coldest of winter? Is it humid and have air circulation?

Also, are you going to place cuttings in the ground, 'summer,' or in pots?

What type of light? Don't forget to start in shade. Especially newly rooted cuttings.

There are some real Gardenia horror stories floating around the net.
Some funny tales, too. Humor in place of regret. lol.

Don't worry about your Gardenia falling apart. Winter is the hardest. If your 'denia survives winter, it should do okay in the future. Getting through the first winter is most difficult, from then on it's fairly simple.

Mites, dry air, and heat are what harms are 'denias.
Thankfully, I have two cool rooms, at times turn on the ceiling fan, and keep humidifers and an indoor fountain running until heat is turned off in spring.
Showers and misting also are important.

Summer, my oldest Gardenia was purchased 1995/6, bought locally.
It's grafted. IMO grafted trees, despite type, are hardier.

Here's a pic taken Nov..

It needs pruning, badly. Don't know when it should be done though, and fear I'll cut bud stems. lol

I know, deep inside, it will never look as nice or flower as much as it did before purchasing. It lived in a warm, humid, green house, w/large fans.

Again, congrats, and good luck, Toni