gpg --list-secret-keys --keyid-format LONG
From the list of GPG keys, copy the GPG key ID you’d like to use. In this example, the GPG key ID is 3AA5C34371567BD2
$ gpg --list-secret-keys --keyid-format LONG /Users/hubot/.gnupg/secring.gpg ------------------------------------ sec 4096R/3AA5C34371567BD2 2016-03-10 [expires: 2017-03-10] uid Hubot ssb 4096R/42B317FD4BA89E7A 2016-03-10
git config --global user.signingkey 3AA5C34371567BD2
or
git commit -S -m your commit message
References
https://help.github.com/en/articles/telling-git-about-your-signing-key
https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work
https://help.github.com/en/articles/signing-commits