My favorite Git client is Tower. I wanted to find a way to sign my git commits despite that not being a supported feature of Tower. Turns out it only took a couple configuration options to make it work.
First, set up your GPG key however you normally do it. I use GPG Tools for OSX, as well as Keybase. Follow GitHub's instructions for adding your GPG key to your account here and here. Once you're able to successfully sign commits from the command line, you can set it up to work with Tower.
Just add `no-tty` to your GPG configuration:
echo no-tty >> ~/.gnupg/gpg.conf
Now when you make a commit from Tower, you should be prompted to unlock your key with your passphrase from GPG Tools, and if you save it in your keychain it should continue to work seamlessly.
Now, whenever you make a commit and push it to GitHub, you should see the "verified" mark next to your commits!