Ive only grown Gardenia jasminoides as a houseplant. They need a bright spot out of direct sun and appreciate being sprayed over as they need humidity. I also used to water mine with rain water as they're not a lover of lime.

They do suffer from scale, mealy bug and red spider mite so check for those first.

then failing any obvious pests give it a dose of epsom salts [magnesium sulphate] as this has trace element magnesium, needed to make the green pigment chlorophyll.

If the new growth is turning a light greenish yellow, it probably has a micronutrient deficiency--magnesium is just one of many, and a complete, acid-forming fertilizer with micros will probably help it. If the older leaves toward the center of the bush are turning bright chrome yellow, it usually indicates watering or drainage problems.