Run:
brew install gnupg gnupg2 pinentry-macgit config --global user.signingkey <key>git config --global commit.gpgsign truegit config --global gpg.program gpg
Followed by:
nano ~/.gnupg/gpg.conf
no-tty
nano ~/.gnupg/gpg-agent.conf
pinentry-program /usr/local/bin/pinentry-mac