source: https://aaronparecki.com/2016/07/29/10/git-tower
Configure your git client to always sign commits:
$ git config --global commit.gpgsign true
Try to sign a commit from the command line before trying it with Tower. Once you're able to successfully sign commits from the command line, you can set it up to work with Tower.
Add no-tty to your GPG configuration, to allow Tower to use it:
$ echo no-tty >> ~/.gnupg/gpg.conf
You'll need to specify the absolute path to the gpg program in order for Tower to be able to find it.
$ git config --global gpg.program /usr/local/bin/gpg
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.