git config --global gpg.program gpg
git config --global user.signingkey ABCDEF123456
git config --global commit.gpgsign true

1. git(GitHub)でGPGを使った署名をおこなう - Qiita
2. GitHubにgpg署名付きのコミットをする - Qiita

wnoguchi@lasthope:~/repos/dotfiles$ git log -n1 --show-signature
commit 88f8c02ddae9253ef28faf6f00ca862ee424ffde (HEAD -> master, origin/master)
gpg: Signature made Sat 06 Feb 2021 07:15:00 PM JST
gpg:                using RSA key EB46F8D643EF3A7CD686C002B4A5CEBBF13A8F59
gpg: Good signature from "Wataru Noguchi <wnoguchi@pg1x.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: EB46 F8D6 43EF 3A7C D686  C002 B4A5 CEBB F13A 8F59
Author: Wataru Noguchi <wnoguchi@pg1x.com>
Date:   Sat Feb 6 19:14:26 2021 +0900

    Initial commit.