it sounds as if it has got too cold. from moving from the growers to the shop or from shop to home. How long have you had it before the browning happened? Often what happened to it over a week ago shows now, so probably nothing you have done.
gloxinia has a corn like a begonia so it will flower again.

Also, when you water plants from below, especially Gloxinias, you don't leave the plant in water any longer than it takes to saturate the surface of the soil. Once it has had enough, about a half hour or so, set the pot on a pad of paper towels for 15-20 minutes to soak up the excess water, then don't water again until the top 5-10 mm of soil is dry. That will take 2-4 weeks, unless the air is exceptionally dry.
When repotting, use a chunky, fast draining mix, such as cactus mix, since Gloxinias like to dry out more between soakings than their cousins, the African violets.

Thank you so much for your answers The leaves are very healthy I'm pleased to say. It's sitting on top of the cabinet in the downstairs loo right now and it's heated in there so it's not chilly. I've not watered it since I repotted it and it's getting quite dry. I've taken on board what you've said about the watering and hopefully it will flower again, even if not until next year.