Plants are yellowing from the bottom up. Do you know your soil pH. Gardenia does not like sandy alkaline soil, nor are they salt tolerant if you live near a beach. Yellowing bottom leaves usually indicate not enough nitrogen. Gardenia is a heavy feeder and needs nitrogen and micros. I think watering once a week in sandy soil may not be enough, especially if it has been recently planted, but if the plant is not wilting it may be o.k.
I would start feeding the plant with a water soluble like miracle grow for acid loving plants formerly known as miracid. It will be the formulation that contains a higher nitrogen and micros like iron. It should give the gardenia a quick boost to correct the yellowing. Have the soil tested, if the pH is over 7.4, I would try to work some peat moss into the soil. If the soil is 7.8 or higher, you may not be able to keep the pH down as the soil will always try to go back toward its baseline and you cannot fight coral or limestone. It is better then to keep the gardenia in a large pot. I prefer 18 gallon or 20 inch pots. Use a good potting soil like MG potting soil (not garden soil). I would add about a 1/4 cup of citrus food to the pot and mix it in well and then after a couple of month feed the gardenia a tablespoon of citrus food when it is actively growing once a month. Citrus food is an acidic fertilizer, it contains sulfur and micronutrients. I use vigoro 6-4-6 the numbers do not need to be high, they just need to be available. Alkaline conditions make nitrogen and micronutrients less available.