Gardenias are difficult plants
to maintain indoors where it is difficult to meet their light, humidity and temperature requirements, particularly in winter. Gardenias prefer bright direct light, high humidity and temperatures at this time of year in the 50's-60's. Any deficiencies in the above requirements can cause leaf yellowing and drop.
Gardenias prefer moist (but not soggy wet) soil, which also provides ideal incubation conditions for mold spores, which might have already been lying dormant in the soil or recently added by airborne spores. Mold generally won't harm your plants
, however, many people are allergic to mold. The usual method of ridding soil of mold is to gently scrape away the mold, along with a 1/2" or so of soil to ensure thorough removal of all the rhizomes and spores. Keeping the soil a bit drier might discourage further mold infestations, however, too dry will be detrimental to your gardenia.
Even experienced gardeners have limited success keeping Gardenias alive indoors in less than ideal conditions, so don't be too discouraged if yours doesn't make it. Probably the best you can hope for is to keep the plant
alive throughout the winter, without expectations of it looking like much, then placing it outdoors again when the weather warms.
Best of luck,