Is this Gift Deed Valid?

My mother 4 months before her death had registered her gift deed. She gifted the flat she lived in to the two younger sons.  the entire process of transfer took place accept one that the letter to transfer of name of the flat could not take place. She had made the letter but dint manage to send it to the societty office. Therefore I wanted to know if the gift deed is still valid.  All the documentation is perfect in terms of signatures etc.

 signature as well as  thumb impression of donor ,both are compulsary or just one of them is enough for gift deed .