Danceswithtiller (love the name!),
Gardenia leaves turn yellow. It's just something that happens. Unless there is a very distinct green vein in the yellow leaves, don't add ironite.
Most people on this forum have a horrible time growing gardenias (do a search and you'll see that) and I think their hard time comes from the gardenias being in a pot. I have one in a pot right now and it's not looking very good.
At least in this part of the country gardenias thrive on neglect. Don't pamper it too much. Make sure the soil is acidic ... if there is any concrete nearby make sure that it has been there for years. Gardenias will grab onto any lime that is leaching out of new concrete and the lime will kill it (ask me how I know).
Other than making sure the soil is acid, I don't think you need to do anything.

Much of the trouble people have growing gardenias is that they are trying to grow them as landscape shrubs in areas where they are totally unsuited. This is compounded by the fact that Home Depot and other big box stores stock them in the outdoor garden center in areas where they are totally unsuited. They are also not suited as houseplants unless one's house is similar to a greenhouse in climate and lighting. Many are doomed to failure and frustration through no fault of their own.

My cousin in Phoenix (where they are recommended outdoors) has had one in a pot on her shady back porch for over twenty years and it gets hundreds of blooms. It is almost four feet tall and wide. Gorgeous.

If you just transplanted your gardenias, some leave yellowing and dropping is its response to the shock of transplanting. The last thing it needs right now are lots of chemicals hitting new roots as the plant gets established. Keep it moist but not sopping wet, and wait at least a month or so before fertilizing.

Jim