Hash algo for Gnupg

[Funaki, Yasuhiro]

Dear expert,

I understand as below when hash is used at decrypting operation.
>Hash algo is used to generate a key to decrypt a private key from
passphrase at decrypting operation.
When above my understanding is correct,
could you advice me how to specify the hash algo at generating key pair
and how to change hash algo after generating key pair?

Regards
Yasuhiro
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