I gave my Gardenia a severe pruning this spring because the leaves looked so bad from the sooty mold. I've been putting used coffee grounds around the base of the plant and so far, the leaves look healthy, there is no sooty mold and the ants are staying away. Mine gets full sun from noon on and it is outside. I don't know how well it would do inside.

I always had trouble with sooty mold on the shaded side of gardenia shrubs in south FLorida. Good air circulation (about 10 cm minimum of clearance) around all sides of the plants helped too.

Make sure they are receiving full sun, especially if close to an irrigation head.

To prevent sooty mold, you need to get rid of the insects on your plant(scale, aphids or mealy bugs usually). Sooty mold grows on the honeydew secreted by the insects.Repeated blasting with water from your hose should get rid of them, or you could use an insecticidal soap.
When you have the insects under control fertilize the plant regularly with a gardenia/camellia/hibiscus type fertilizer, water well and give an occasional application of epsom salts (this is for Florida's nutrient deficient alkaline soils - not sure what type of soil you may have?).
Gardenias like a good amount of sun - perhaps light shade in the middle of the day. I think that growing them indoors would be a lot of trouble and in your zone 11 garden they would be much happier outside.
The key to prevent sooty mold is to get rid of the insects and then a healthy well-fertilized, well-watered plant will have less problems.

Again, to prevent aphids and other sucking insects that leave honeydew, you need to ensure a sun loving plant is getting enough sun. ENough sun means strong new growth that is not "weak" and preyed upon by as many aphids, etc.

Even other sun-loving plants like oleanders, mandevilla and kalanchoe flower stems will be covered in aphids if sunlight is not strong enough.