Put it outside if its warm enough. Dont water it so much depending on the depth of the pot you could be giving it root rot. Let the soil dry out some. I got an azalea once and had the same problem after it died i pulled it up and there were no roots. So id def say get it outaide if its warm enough and stop watering it so much.

Probably over-watered. Gardenias like humid air and good light. If there is not enough light then the plant does not need much water. The excess water causes the roots to rot. When you water does it come out of the bottom freely? If not then the bottom hole may be clogged.

The roots need air too. I would start by poking holes into the dirt using a chopstick. May 10-12 holes all the way to the bottom.

If you are feeling a bit brave, take it out of the pot carefully without disturbing its root mass and check if its soggy and may be even smelly. If so then wrap the soil in tissue paper. The paper will absorb excess water. Then prune some of the rotting roots from the bottom and repot the plant with some dry bonsai soil.

Small bonsai is best watered by putting the pot in a tray of water deep enough so that the soil soaks up from the bottom. Takes about 15-30 minutes. Then take it out and let the excess drain away. Do not use any fertilizer now. Just let recover first.

Gardenias are fussy plants. They usually do better outside than in homes so if your temperatures will allow it I'd probably let it stay outdoors.
You might find the below link to a conversation about gardenias helpful. There are some tips there about how to grow them indoors. It's tough but it can be done.

Here is a link that might be useful: Gardenias Indoors

I have few dwarf gardenias that are doing well in pots. By well I mean they produce enough flowers for their size a couple of times a year. In the summer they are outside and in the winter they are under my lights in the basement. They could have done better if I had given them better soil and regular fertilizers.

I read the link above and I am thinking why are not my gardenias not as fussy. It could be that dwarf ones are less fussy. Really, I do not do anything special. I water them every 3-4 days to make sure they are not dry. Mist them sometimes. Otherwise they seem to do just fine. The buds are swelling up now and in a month or two they will bloom. They are very slow may be because it is probably very root bound now. I will have to repot many plants in the next few months.