No need for homebrew or anything like that. Works with https://www.git-tower.com and the command line.
- Install https://gpgtools.org -- I'd suggest to do a customized install and deselect GPGMail.
 - Create or import a key -- see below for https://keybase.io
 - Run 
gpg --list-secret-keysand look forsec, use the key ID for the next step - Configure 
gitto use GPG -- replace the key with the one fromgpg --list-secret-keys 
git config --global gpg.program /usr/local/MacGPG2/bin/gpg2
git config --global user.signingkey A6B167E1 
git config --global commit.gpgsign true 
- Optionally configure annotated tags to be GPG signed
 
git config --global tag.forceSignAnnotated true
- Add this line to 
~/.gnupg/gpg-agent.conf 
pinentry-program /usr/local/MacGPG2/libexec/pinentry-mac.app/Contents/MacOS/pinentry-mac
- Add this line to 
~/.gnupg/gpg.conf 
no-tty
% keybase pgp export
% keybase pgp export -q CB86A866E870EE00 | gpg --import
% keybase pgp export -q CB86A866E870EE00 --secret | gpg --allow-secret-key-import --import
% open https://github.com/settings/keys
% keybase pgp export -q CB86A866E440EE00 | pbcopy
- https://github.com/pstadler/keybase-gpg-github
 /usr/local/MacGPG2-- this is where MacGPG binaries live- https://gpgtools.org
 - https://www.git-tower.com
 
why not just
/usr/local/bin/pinentry-mac? And whywhich gpg? I thinkgpgon its own should suffice.