Hi
1) According to Hindu Adoptions and Maintenance Act, 1956, after adoption, the adopted son/daughter lose all the rights of a son/daughter in their biological family, including the right to claim any share in the estate of the biological father or relations, or any stake in the coparcenary property.
2) Also, in your case, the grand father is still alive and hence the property has been actually gifted by your grand father to your father .
3) Hopefully the gift deed (executed by your grand father to your father) is a registered gift deed. If there is no registered gift deed, your grand father will need to come to court to testify that he has executed the gift in favour of your father.
4) Since the property is hard earned property of your grand father, it is his self acquired property and as such your grand father is right in gifting the property to your father.
5) People generally confuse between ancestral property and grand father's property. If the property acquired by grand father out of hard work, then the property is a self acquired property of grand father and the grand father can do any thing (Execute a gift/WILL/Sale) of the said property during his life time and his son's, daughter's cannot claim any share whatsover from the said property.
Hope this information is useful.