Last updated March 13, 2024
This Gist explains how to sign commits using gpg in a step-by-step fashion. Previously, krypt.co was heavily mentioned, but I've only recently learned they were acquired by Akamai and no longer update their previous free products. Those mentions have been removed.
Additionally, 1Password now supports signing Git commits with SSH keys and makes it pretty easy-plus you can easily configure Git Tower to use it for both signing and ssh.
For using a GUI-based GIT tool such as Tower or Github Desktop, follow the steps here for signing your commits with GPG.