What would make my new healthy looking gardenia bush wilt after it was planted in my garden?

Transplant shock is most likely the problem - basically your bush is in shock. This stress can be caused by different things taking place including not watering it in well and keeping it well watered (but not wet!). Newly planted shrubs, trees, plants, etc., require more water after they have been transplanted (make sure the area is well drained so as to not deprive them of oxygen and drown their roots and them). Transplant shock can manifest itself in wilt, yellowing and/or dropping of leaves, and even stunting of new growth. Transplant shock can last for quite a long time in some plants, again depending on the severity. Do not fertilize right now! This will only worsen the situation by making the plant grow more during a time of stress. Just be patient and give the plant time to adjust.