It sounds like you are knowledgeable  enough about gardenias to look for the obvious pests and how to treat them.  So I am going to assume that you have looked at the undersides of the leaves carefully for any pests, though in winter those should be unlikely.  I just looked at my gardenia and found some brown leaves, similar to yours, but not to that same extent. My best guess is that the extreme cold we had that followed some warm weather caused some cold damage to the plant that turned the leaves brown.  You can’t know about the overall health of the plant until new growth begins in the spring.  If that new growth looks different, then you may have something more than cold damage.  The damage can be cut out if it affecting an entire branch.  Otherwise I would just continue to monitor and wait until spring.