The registered gift or settlement deed cannot be revoked unilaterally either by the donor or by anyone, if at all the cancellation has to be done it can be done with the consent of the donee that too by both the donor and the donee executing a registered cancellation deed. If the registrar do not agree to this then they may have to approach court of law for this.
Thus the registered gift settlement deed cannot be cancelled for the reasons stated by you, however as per the condition your grandfather can enjoy the property till his lifetime, further if the beneficiary happens to die during the lifetime of the donor, then the property shall automatically stand transferred to the legal heirs of the deceased beneficiary/donee.
There is no such law that the three sons are entitled to your grandfather's property equally during the lifetime of your grandfather. It is his own discretion and desire to settle his own property in favor of his one son alone, nobody can question this decision as a right.