1. Please clarify whether 'D' has got BBMP A Khata or entry in BBMP B Register in the first instance.
2. Registration of GPA is a must only subsequent to Supreme Court Judgement of 2011 and it will not apply to pre-2011 cases.
3. Since 'B' registered Sale Deed in favour of 'C' and all statutory payments made to State Government in 2003 itself, the title to 'C' is in order.
4. Moreover Khata was mutated in favour of 'C' by BBMP way back in 2003 itself, i.e., before the Supreme Court judgement of 2011 and the registration of GPA was not compulsory in the year 2003 and the GPA entered into after 2011 has to be compulsorily registered.
5. If the Gift Deed executed by 'C' in favour of his brother 'D' is duly stamped and registered before the Registrar/Sub-Registrar, then it is in order and the 'D' will have absolute right to transfer the property to any body.
6. At present if the Khata is in 'D' s name and mutated in his name in the revenue records, then you can proceed further, subject to getting the documents verified by a local Advocate.
7. It is true that unregistered GPA is not valid, if it is/was executed subsequent to Supreme Court judgement of 2011.