Annie,
It is totally normal for gardenias to yellow in leaf this time of year. Do nothing.
Stop feeding them. It's fall, they don't need food now.
They look normal.
Just make sure they are mulched for the winter, and next spring when it warms up put some composted cow manure around the bottom of them and then remulch.
Congrats, they look good.

I would like to share my experience in growing gardenias.

I grew my first gardenia from a rooted cutting given to me. I have never read into gardenias before and have never followed what information was available on the internet.

So then I've been growing the cutting on normal soil. Feeding with normal fertilizer, none of that ericaceous stuff. Nor have I ever done anything to increase the acidity of the soil.

My plant never had any problem with chlorosis whatsoever. It grew well and it's been with me for more than two years and have been constantly blooming. Only complaint I have is that it's fragrance is not to my taste - most of the time an oily scent that smelled a little like crayons.

So overall, I feel that there has been an over emphasis on providing gardenias with acid soil, acid fertilizer.

My recommendation is to provide your plants with normal fertilizer and see how it goes. In growing season, provide with lots of fertilizer and lots of water.

But I do notice one thing on your plant growing surrounding. Rain and water that washes down from that concrete wall would tend to decrease acidity towards a more alkaline condition. Something you might like to consider.

Let me share a photo of my gardenia. It's a variegated species. Photo taken today.

Ok just one more.

Beautiful photos Tolip and Annie one of my favorite plants all mine are grown in pots and i did have a few problems early on in the year with the leaves looking a little like yours.
Some one of here mentioned banana skins and they have worked a treat for me! i either chop them into small pieces put them in a bucket with some rain water and leave to soak for 48hours then feed to the gardenia or put them in a blender then mix with water when i feed the plants
so you could give that a try but it is more likely to be as butterfly4u said