Purchased a property which was passed through a gift deed not aware of

My father in law had purchased 3 sites in gorevigere area of bangalore in 2006 on behlaf my husband. The original owner of the property had made a GPA in the name of mr. X who made a gift deed of tge property to mr. Y. This mr. Y sold the property to one mr. Z from whom my fath in law purchased. Now my father in law was unaware of the GPA AND GIFT DEED between the previous owners. So now when we want sell the land we came to know that we cannot sell it because of the gift deed and GPA. Please help me out here as to what can be done.