Gardenias prefer acidic soil. Be sure to be mulching w/ pine straw or other acidic mulches.  Also, iron deficiency can cause yellowing.  (This is because iron cannot be taken up if pH is too high.)  You are right that some yellowing can be due to normal senescence. Check new foliage for health. Clemson, https://hgic.clemson.edu/factsheet/gardenia/,  has a good article discussing yellowing and fertilizer requirements.  Check it out for more info.