If you received a flat in a co-operative society via a gift deed executed through a power of attorney without the society's prior consent, and the original owners are now unavailable, you have legal avenues. You can issue a legal notice to the society demanding membership based on your registered gift deed. If this fails, consider filing a 'Declaration Suit' in civil court to establish your title and compel the society to grant membership. You may also need to involve authorities like RERA and the Registrar of Co-operative Housing Societies.