Property law + inheritance

 Since seller is absolute owner of property so he can execute sale deed without consent of his sons,daughters grand children 

( 2 ) you need not to get seller legal heirs and dependents sign on sale deed as seller is absolute owner of property.

If possible, insist for the witness signature of his son to avoid problems.

If the gift deed is properly stamped and registered you are the absolute owner and can sell the property. You do not need any permission from anyone. 

@Kishor Mehta,  Gift deed is properly registered and executed by son(A) to his father(S) name. Father (S) is the seller in my case.

Since A is the absolute owner of the property via Gift deed, he can execute sale deed to you.

There is no need to have A's legal heirs as witness over the sale deed.

A is having a right to revoke (cancel) the Gift deed whether it is not valid in a court of law.  But the SubRegistrar cannot refuse the registration of revocation of Gift deed.  So it is best to get the sign of A in the sale deed.

There is no problem in buying the property in the given circumstances / facts.

However, for your further satisfaction it is better to consult and engage a local prudent lawyer for appreciation of facts/ documents, professional advise and necessary proceeding.

Yes, the title to the property is clear. The donor cannot revoke the gift nor  can the legal heirs dispute the sale.