Validity of Notarized GPA on Immovable Property in Karnataka 1996

Hi, My father purchased a immovable property in Bangalore Village Panchayat via GPA during 1996. The property was allotted by Govt of Karnataka in 1972 to the owner who belongs to Schedule Caste. As per law the owner didn't have rights to sell for 25 years of term from the day of allotment from Govt. Hence a GPA was executed via Notary where we have paid final amount. GPA was signed by Owner/ his wife/ Son in front of Notary. The property owner passed away in 1997 & my parents were not aware of importance of Registration during those days. Finally a Notarized promissory note was signed by owner's son & wife declaring they have received money & Property was sold to us saying we enjoy full rights in 1998 after the term of 25years. We own full documents including the Hakku patra issued by Govt of Karnataka in 192, then paying tax from 1996 onwards regularly. In 2008 property came under BBMP from Panchayat & we have been in possession from 1996 onwards till date. Could you please let us know if we are entitled to hold the property in future without any riddles or to sell the property to a third party ?