Determine your key ID
gpg --list-secret-keys --keyid-format LONG
/Users/davidcondrey/.gnupg/secring.gpg
--------------------------------------
sec 2048R/YOUR-16-DIGIT-KEY-ID YYYY-MM-DD [expires: YYYY-MM-DD]
Your ID is a alphanumeric 16-digit code following the first forward-slash.
Define your key ID in your git config
git config --global user.signingkey YOUR-16-DIGIT-KEY-ID
As of version 1.7.9, git commit accepts the -S option to attach a signature to your commits. Using this option will prompt for your GPG passphrase and will add your signature to the commit log.
git commit -S -m "Your commit message"