Certified GKH Gardening Expert
Your minimum winter temperatures on the USDA plant hardiness map in Jacksonville dips below freezing (zone 9a; 20 to 25 degrees F). But online weather maps say average temps in Jacksonville rarely get below freezing in winter. Gardenia is generally hardy in zones 8 to 11, but there are more cold hardy varieties now.
If your experience is that Jacksonville stays above freezing this time of year, go ahead and plant it. If you have doubts, go ahead and buy it, but keep it inside or in the garage till all danger of frost has passed. Then plant it outside.