It would be a good idea to have a soil pH test performed on a sample of soil taken from the area where the gardenia is to be planted.  A pH between 5.0 and 6.5  would be ok for gardenias.  If the soil pH is above 7.0 it will be a challenge to keep gardenias healthy.

If possible, consider amending the planting bed with compost, peat moss or manures to improve the nutrient and moisture-holding capacity of the soil.

The University of Florida also makes these recommendations for planting gardenias:

Once an appropriate location has been selected, dig a planting hole two times the diameter of the container that the plant is in. The hole should be as deep as the root ball.

• Remove the plant from the container while being careful not to disturb the roots or root ball.
• Place the plant in the hole, making sure the top of the root ball is even with or slightly above ground level. Adjust the plant so it is level and properly faced.
• Fill soil around the plant until the hole is half full. Fill the hole with water and allow this to settle, then finish filling the hole with soil.
• The surplus soil removed from the planting can be used to build a dam 2 - 3 inches high around the outside edge of the hole, forming a saucer that will help retain water.
• Apply a 2 - 3 inch layer of mulch around the plant. Be sure to keep the mulch a few inches away from the base of the plant to prevent fungal problems. Replenish mulch as needed.
• Water a minimum of twice a week during the first six weeks of establishment.

Additional information on care of gardenias can be found here:  http://edis.ifas.ufl.edu/mg336