Certified GKH Gardening Expert
As long as temps don't fall below 20 F. for extended periods, gardenias can survive outdoors just fine. If it falls below this, the plant will need to come inside for winter or placed in an unheated garage. If left outdoors, I would, however, recommend placing it in a sheltered location to help protect it during any unusual cold spells and you may even want to wrap the pot in burlap, bubble wrap, etc. Also, in the event of heavy frost or freeze warnings, you may want to cover it.
Here is more info that should help: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm