Mel, are the brown spots little bumps that can be scraped off or are they embedded in the leaves?
If they can be scraped, it's probably scale bug. Are you familiar w/this insect?
If that's the problems you need to rid them or they'll kill your gardenias..
Watering daily? Isn't that too much? Soil should dry a bit between waterings.
How tall are your plants? If they're established, they don't need all that water..One good drink then let soil dry a bit.
Feeding during growing season is important, too..You need an acid type fertlizer, but feeding should be done during growing season.
Overwatering will cause leaves to yellow. So, do scale. Inspect your plants and see if these spots rub off..Toni