You can challenge the gift deed only on the ground that it was made by your father under duress by the donees. Since the medical certificate is not required to be attached along with the gift deed there has to be some circumstantial evidence to prove the practice of duress. If he was hospitalized just before the gift deed was made and he was incapacitated to either sign the gift deed or understand the legal import of what he was doing then this is a suspicious circumstance. You would require the hospital records of your father to prove that he was incapacitated.