If your gardenia plants are turning yellow on the leaves it's usually caused by Chlorosis since gardenia plants are prone to chlorosis.
To fix the yellow leaves on the gardenia plants you should increase the humidity in the room where the gardenia plants are.
And also use some fertilizer with iron chelete in it that is water soluble.
Use the water soluble fertilizer mixed at half strength every two weeks for the gardenia plants and make sure they're getting enough water as well.
The leaves should turn back to their original color by doing the above.
Also another reason that gardenia leaves turn yellow is due to lack of sunlight so try to get the gardenia plants some more natural sunlight or increase the artificial light for the gardenia plants.
Gardenia Plants need quite a bit of light and natural sunlight is the best.
So you can try placing the gardenia plants near a window that gets a lot of sunlight exposure.