Check the tag to determine if they are one of the cold hardy varieties, such as:
'Chuck Hayes' Zone 6b-10
'Grif's Select' Zone (5)6-10
'Klein's Hardy' Zone (6b)7-10
'Shooting Star' Zone (6b)7-10
The latter 3 are single-flowered varieties, only 'Chuck Hates' has double-flowers.

To successfully grow Gardenia's, you must provide a site that is moist, but WELL drained and shielded from afternoon sun. This applies to winter, as well as summer, plus protection from cold winds is necessary in the winter. Mulch heavily during the first 2-3 winters after planting. Apply the mulch before the first freeze in the fall. Planting near a brick or stone building/wall will also benefit them in the winter, but care must be observed in summer, so that they aren't subjected to reflected heat.
Soil must be very acidic, in the pH 5-6 range.
Do not fertilize them in the fall. Apply a slow release type in spring and midsummer.
In your Zone, avoid planting Gardenias in the fall. A high failure rate usually results.
You may be able to grow the standard Gardenia augusta (jasminoides) and the low growing cultivar, 'Radicans'. if winter protection is provided in the form of a burlap wrap or with a "shrub coat". I have grown those for more than 20 years, with no winter protection and they have suffered very little winter dieback.
Avoid the variegated foliage cultivar, 'Variegata'. Those are best reserved for south Florida!
Happy gardening!
Rb