First, your comment that its still snowing - don't even think about cutting back the gardenias until you are more or less certain that winter, and its cold temperatures, has passed, so don't do it now. You need to wait till spring starts properly, whenever that will be, without the risk of sudden cold snaps recurring.
When you've got to that point, check the bushes over again. You're right when you say to cut back to healthy growth, but sometimes these shrubs can be killed right back to the ground. There seems to be dead growth all the way down, with evidence of new growth higher up the stems, so you have a choice - you either cut them right down, or cut back to where you see new growth coming, as low down as possible. This unfortunately does mean no flowers this year, but there's not much you can do about that, other than be thankful they're still alive after such severe cold.
If you have to cut right down near the base, or fairly close to it, make sure you give them a good feed as often as recommended on the product you use, but do not feed past the end of June or early July. Feeding after that time just encourages new growth to form, making the plant more vulnerable to cold weather, because that growth won't be hardened sufficiently if winter arrives early,