1. The Hon'ble Supreme Court judgement prohibiting the use of GPA for transfer of property came in 2011. In the instant case, Mr. A has given GPA to Mr. B on [deleted]. In other words, the GPA executed by Mr. A in favour of Mr. B is much much before the Supreme Court judgement and hence it will be valid.
2. The Buyer is advised to get the property documents vetted by any Lawyer before proceeding further.