You may leave Garni either by marshrutka, bus or possibly taxi. If you get stuck in the village past the time that the minibuses stop traveling to and from Yerevan, you need not to worry. Talk to almost any villager and they will most likely be willing to help you in any way possible and will try to arrange a taxi for you to get back to Yerevan. It will probably not be an "official" taxi but will charge comprable rates.