Given you have created a pull request (PR) with a single commit on GitHub. The commit is not GPG-signed. The maintainer asks you to sign the commit.

If you are the only one working (committing) to this PR, you can force-push a signed commit.

# Sign the latest commit on your local repository
git commit --amend --no-edit --gpg-sign

# Force push the commit to your remote repository (fork)
git push --force-with-lease

GitHub will automatically update your PR.

gpg sign pushed commit