When receiving a residential flat as a gift from your parents, registering the gift deed with the Sub-Registrar is mandatory. The stamp duty payable varies significantly depending on the Indian state, with some states charging a fixed nominal fee (like Rs. 5,000 in Karnataka or Rs. 200 in Maharashtra) while others may charge a percentage of the property's value. It's advisable to confirm the exact stamp duty and registration requirements with your local Sub-Registrar's office or a legal professional.