Gardenias are sensitive to cold, low humidity, and improper watering. Any of these could be the cause of leaf drop on your plant.
Avoid pruning your plant in mid-winter. It's better to do so right after bloom, when the plant is actively growing. If it has suffered some dieback due to cold weather, wait until late winter when growth begins, and trim off the dead sections at that time. Otherwise, gardenias need little pruning. You can simply shape them to keep the branches uniform in length.