If your grandfather is desirous of transferring this amount to you out of love and affection he can do so. for this there is no necessity to execute a gift deed.
Or if he is intending to give it to you as a loan, then you can execute a demand promissory note in his favor promising him to repay the amount at the time of his demand at a later date.
Since this is your grandfather's own money, he need not ask permission or consent from his own children or anyone to transfer this money to you.
He can ignore the objections raised by his sons in this regard and can proceed with his decision to give you this amount either as a gift or as a loan.
