Property sold by Registered GPA Holder on July 1999

Hi All, I need all your help regarding Immovable Property Legal Issue. My father had purchased a plot in Bangalore on July 1999. It was purchased from a GPA holder who had authority to sale the land on behalf of the main land owner. GPA holder is a Registered GPA holder. But we have Registered the property at the time of purchase from the GPA holder. I recently took Encumbrance Certificate from Sub Registered Office. As per the EC the said property belongs to us. Let me now explain the problems we are facing. Now a builder has purchased the whole land from the main land owner who transferred the plot to us through GPA. Our plot is 30*40 Sq Ft measurement. But the builder has purchased own land of 2.5 acres. Now the builder is saying that whole land belongs to him. Our plot is one among the land. Now builder is claiming whole land. Even GPA holder who transferred the plot to us is not responding properly to us. I have heard that the GPA holder can transfer the land with proper Sale Deed to others. And recently Supreme Court as on October 2011 has barred the transfer of properties through GPA. The above court order is not retrospective. That means properties transferred from GPA Holder is still is valid if done before October 2011. Please suggest me whether the above CASE holds strong if I go against the Builder in the courts of law. Please help me as soon as possible.