you can you ssh-private key(also with passphrase) instead password

If you specify ssh-private-key instead password gitcrypt will you it for encryption.
Actually it will you just part of private-key.
If private key is protected with passphrase - it will be requested and TEMPORARY stored in .git directory with permission 600. It will be erased after timeout(default is 30 seconds). This timeout is extended on each access to this temporary key. After timeout passphrase will requsted again if needed.

Also you can recrypt old repo with new feature by: "gitcrypt recrypt" and specify ~/.ssh/id_rsa or any other private key file.

Sorry for my english