Like all the other pelargonium varieties, these will not survive a winter outside - they are able to withstand frost up to -10degC, but what they can't take is the combination of damp and frost, they get a fungal infection and rot.
You will need to reduce the topgrowth and move the plant to somewhere frost free but not heated, with natural light, and keep watering to an absolute minimum (a dribble once a month or every six weeks at most). Helps if you dust the plant with sulphur dust, and make sure you remove any fallen leaves as soon as they drop to discourage fungus infection, to which, in a non airy space, they're inclined to succumb. The sulphur dust helps to prevent fungal infection.
Repot in about March and move to a warmer, brighter place, increase watering, start feeding when you notice new growth, ready to move outside at the end of May when all frost is past.