A. When the owner of property transfers it willingly, it is voluntary alienation. It may be made in three ways, (i) for consideration, e.g. by sale, mortgage, lease or exchange, (ii) by gift, and (iii) by will. So in your case, if the landowners executed a GPA by way of registration for the purpose of the alienation of the property. In such being, it is a complete violation of the Government norms and the GPA will be invalid. But, if the GPA was executed only to manage and maintain the land, then only GPA would be valid.
B. After the execution the GPA, Confirmation deed was executed by the landowners with respect to the land. Hence, land's confirmation deed would be valid.
C. In your case, the Sale Deed was executed in the year 2004, hence it will be valid subject to A point, kindly contact local advocate to know the perfect answer by showing all legal documents.