You will have to get the GPA attested by the Indian embassy or consulate in that country.
Any Notary in USA can also attest it for you. You don't need witnesses for attestation purposes
The Grantor i. e. you should sign the GPA on all
the pages and can send it over to your father.
Finally your father should get it registered from the local Sub-Registrar office or Sub-Divisional Magistrate office in India, by paying the appropriate registration charges applicable in that State, in presence of 2 witnesses.
Any any doubts, feel free to contact.