Most gardenia plants are not hardy and cannot be left in the ground over the winter. Unless you know for sure that your variety is hardy, we would not recommend that you plant it outdoors.
They do love to be outdoors, however. Be sure it has good drainage and the soil is not allowed to go completely dry. The top 1/2" can dry out between waterings. They enjoy humidity. They can react to the transition to a new environment by dropping buds. Buds do best with a steady temperature of 62-63 when forming. Otherwise, 60-75 is fine.
Gardenias like bright, but not direct, sunlight. They like acid soil.