Is it obligatory to repeat the ghusl when water becomes available again, after having performed ghusl with two liters of hot water due to a water قطع and cold weather, knowing that the obligatory prayer has already been performed?

If the head and body are covered with water, then the ghusl (ritual bath) is valid, and the prayer is valid, no matter how small the quantity of water. The Prophet, peace and blessings be upon him, performed ghusl with a saa' (a measure of volume), and there is no minimum limit for the water used in ghusl. Rather, the criterion is to cover the entire body with water, even if it is merely by flowing. Certainty in covering the entire body with water is not a condition; rather, a strong presumption is sufficient to alleviate hardship and difficulty.