Dear Cookie, Gardenia plants bloom stronger on younger, newer wood. Keeping them alive and strong in the winter inside is a challenge. Lots of light, high humidity, some regular acidic fertilizer and even moisture in the soil without keeping the pot too wet. You can prune both new and old wood if you need to. Prune when you know the plant will respond well. If the plant is barely growing indoors, you may want to wait until it is back outside and growth resumes before pruning. If actively growing (new leaves, flower buds) you can most likely prune it then. If it was outside for the summer, be on the watch for insect outbreaks from Spider mites and scale insects
Sincerely, Richard Hentschel Sincerely, Richard Hentschel