1. Any adult person with sane mind can sign as witness to any deed. 

2. The radiation of the witness to the parties of the deed has no significant ce in the eye of law.

3. Therefore if your father sign as one of the two witness then the proposed gift deed would not suffer from any legal infirmity. 

Yes, as per law witness main motive is to see the signature in front of him by both the parties. He has no rights to check the documents.

Its better you keep an independent 2 witnesses for the same. but they are not present then any witnesses will do.

Section 123 of Transfer of Property Act 1882 says  For the purpose of making a gift of immovable property, the transfer must be effected by a registered instrument signed by or on behalf of the donor, and attested by at least two witnesses. Executant can sign in the presence of one of the two witnesses and mark in the presence of the other ,so that the other witness is aware of the signature of the executant.It is essential that the witness should have put his signature animo attestandi, that is, for the purpose of attesting that he has seen the executant sign or has received from him a personal acknowledgement of his signature. 

Your father can put the signature as witness in the gift deed.

yes that can be done there is absolutely no legal hurdle in that.

There's nothing wrong in your father becoming an attesting witness to the registered gift deed executed by your grandfather in your favor. 

1. Yes your father can be legally witness to the gift deed between you and your grandmother.

There is no bar to your father being witness to gift deed 

Yes, your father can be a witness. 

Any person of sound mind, and above the age of 18 can be a witness. 

Yes your father can be the witness 

any least two witnesses should attest in the gift deed . 

- Since, the said property is self acquired property of your grandfather , then he is having his right to gift you without the interfere of any other legal heirs like your father etc. 

- Yes, he can be one of the witnesses in the said gift deed.

- Further , as per the Section 17 of the Registration Act, the registration of a Gift deed is mandatory . 

1. Yes your father can be witness in the gift deed and it is legally valid. 

2. Any person with sound mind above age of 18 years can be witness in a gift deed. 

Any person can be witness to gift deed.

.is it legally valid for my father to be as a witness for the gift deed between my grandfather and me? - YES. 

Dear Sir/Madam,

It is suggested that  you father can become the witness and there is no issue in that.