How bright is the spot? How far from the window, and how high and wide is the window? I find that the most common reason why buds drop on Florist Gloxinias is that people tend to put them in decorative situations where they do not get very bright light. I would put it under lights or right by a bright window. Keeping it lightly moist is also important, but it may be that your plant has already begun the process of aborting its buds. Hopefully enough will open and some new buds will set before the growth goes back to a vegetative stage. Second to not enough light, drying out is another reason why gloxes abort their buds.

The window is quite big, i'm not sure about the measurements...the plant is a couple of feet away so it gets plenty of bright light. The buds haven't dropped they're just brown so I cut them off.

ooooh this is sooo fustrating! out of 15 buds I have 4 now. These 4 are a little bigger and haven't turned brown yet. I hope they hurry up and open before they mess up.

Florist gloxes often experience bud blast, because of changes in their cultural environment, including moving to lower humidity, or due to any of a number of reasons including alterations in airflow, light, watering etc. These plants likely came from a greenhouse where they set buds under semi-optimal conditions, then were moved to poorer conditions in the florist shop. So the bud blasting may have been getting underway before you took the plant home.

If it's any consolation, under good home care the next round of blooms will probably be better.

Eric put it perfectly. I've blasted buds, just moving a plant from one location to another. I usually look to less light as the culprit, but it could be any combination of the factors Eric mentioned. But, look forward to the buds you produce from your efforts on its next sprout from the tuber.