The rules of gardenias = acid soil, humidity and protection from drying wind, morning sun only. SoCal = alkaline water, drying wind, intense sun. So unless you are creating a very specific protected environment for your gardenia in Southern California you are going to struggle.
The challenge is that you notice the foliage drying and you want to overwater - which in turn stresses or kills the roots. The solution is to NOT overwater, but to protect the plant from drying.
Gardenias, azaleas and camellias all have similar needs, so once you learn how to manage one in SoCal conditions, you just need to apply the same care to all. Even "sun type" azaleas will need protection in SoCal. And you absolutely will need to use soil acidifier - the irrigation water in SoCal is a killer to acid-loving plants.