Steve on 21 May, 2017 09:08 AM
Do you still have access to your old mac? If so, you may want to do a search for 'gnupg' in macOS keychain and see if that shows any results.
Should you be unable to get back to your password and have no backup or idea what it could be and have access to the old mac, you could try to revoke your key on that old mac and create a new key on the new mac.
Steve on 24 May, 2017 11:23 AM
thanks for the updates. Great news to hear you were able to retrieve the password from your old mac. Please consider storing it in a secure location (bank vault, password manager, hidden place in your home, ...) so you don't run into this situation in the future.
If your key expires soon, please re-new the expiration date.
Should your subkey be expired, please create a new subkey.
navigate to the subkey tab
click the "+" button below
confirm with "Generate subkey"
Double click your sec/pub key in GPG Keychain and then you'll see a "Change" button next to the expiry date.