I usually use the microwave to reheat the gulab jamuns and sugar syrup. They get perfectly soft and spongy. Transfer the sweet to a wide microwave-safe bowl and heat for 5 - 10 minutes at high power. Otherwise, bring them out of the fridge, allow them to sit at room temperature, and then transfer them to a saucepan along with sugar syrup. Cover the pan with a lid and reheat them at low flame for 5 - 10 minutes. But keep checking them while reheating to avoid burning.