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You can prune it now but you will not have any blooms next year if you do so. The plant won't be harmed, but the display it puts on next year will be.

Prune them now. If you prune them now, you should be able to save them. When you transplant a bush, especially a large one, the roots get pruned because you can't get them all out of the ground. There is then more leaves and branches than the roots can support. Pruning the branches, even now, will bring that balance back in line.

If it is potted, you can fertilize with a water based fertilizer every few weeks. In terms of repotting, keep an eye on the bottom of the pot and when you see roots beginning to peek out, it is time to repot. Be aware that repotting will stress the plant and should only be done when necessary and when you do it, expect it to drop its blooms and possibly a few leaves.

Gardenias are particularly toxic to cats. If a cat has eaten a gardenia and has not already begun to vomit, the cat needs to be induced to vomit and immediately taken to the vet.

These articles will help you: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/helping-a-gardenia-bush-with-yellow-leaves.htm
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-care-to-get-a-gardenia-bush-to-bloom.htm

Most gardenias cannot survive outside in areas that drop below 15 F in the winter. If you live in an area that gets colder than this in the winter, you will need to bring them in. If you need to bring it in, dig the shrub up from the ground with the rootball being as wide as the widest part of the canopy. I would trim back the plant at this time too, to help it adjust to a smaller root system. Then acclimate it for bringing in the house. This article will help:
https://www.gardeningknowhow.com/houseplants/hpgen/using-pesticides-and-other-chemicals-on-your-houseplants.htm

Since you stated that you have fairly mild winters with the occasional cold snap, mulching and wrapping the plant in the winter should be enough to help protect it against the cold.