Registration of property purchased on GPA

Dear Sir/ Ma'am, My mother purchased a flat through GPA in the year 2000 however the land on which construction was held was freehold and the deed are also registered. All the GPA and SPA papers are registered under sub-registrar office and in that papers the power to execute sale deed is also given. My question is how we can convert it into sale deed since the seller is not available right now and he also purchased a flat at GPA only. Whether my mother can execute gift deed in favour of my brother so that it becomes registered property?. Please provide solution for that. Thanks