gpg failed to sign the data fatal: failed to write commit object [Git 2.10.0]

I ran into this issue with OSX.

Original answer:

It seems like a gpg update (of brew) changed to location of gpg to gpg1, you can change the binary where git looks up the gpg:

git config --global gpg.program gpg1

If you don't have gpg1: brew install gpg1.

Updated answer:

It looks like gpg1 is being deprecated/"gently nudged out of usage", so you probably should actually update to gpg2, unfortunately this involves quite a few more steps/a bit of time:

brew upgrade gnupg  # This has a make step which takes a while
brew link --overwrite gnupg
brew install pinentry-mac

on old homebrew:

echo "pinentry-program /usr/local/bin/pinentry-mac" >> ~/.gnupg/gpg-agent.conf
killall gpg-agent

On more rencent one like M1 macs:

echo "pinentry-program /opt/homebrew/bin/pinentry-mac" >> ~/.gnupg/gpg-agent.conf 
killall gpg-agent

The first part installs gpg2, and latter is a hack required to use it. For troubleshooting, see this answer (though that is about linux not brew), it suggests a good test:

echo "test" | gpg --clearsign  # on linux it's gpg2 but brew stays as gpg

If this test is successful (no error/output includes PGP signature), you have successfully updated to the latest gpg version.

You should now be able to use git signing again!
It's worth noting you'll need to have:

git config --global gpg.program gpg  # perhaps you had this already? On linux maybe gpg2
git config --global commit.gpgsign true  # if you want to sign every commit

Note: After you've ran a signed commit, you can verify it signed with:

git log --show-signature -1

which will include gpg info for the last commit.

If gnupg2 and gpg-agent 2.x are used, be sure to set the environment variable GPG_TTY.

export GPG_TTY=$(tty)

See GPG’s documentation about common problems.

If everything fails, use GIT_TRACE=1 to try and see what git is actually doing:

$ GIT_TRACE=1 git commit -m "Add page that always requires a logged-in user"
20:52:58.902766 git.c:328               trace: built-in: git 'commit' '-vvv' '-m' 'Add page that always requires a logged-in user'
20:52:58.918467 run-command.c:626       trace: run_command: 'gpg' '--status-fd=2' '-bsau' '23810377252EF4C2'
error: gpg failed to sign the data
fatal: failed to write commit object

Now run the failing command manually:

$ gpg -bsau 23810377252EF4C2
gpg: skipped "23810377252EF4C2": Unusable secret key
gpg: signing failed: Unusable secret key

Turns out, my key was expired, git was not to blame.

I've DONE it through this short and easy recipe:

Auto-sign commits on macOS (Globally and with different IDEs):

Get your signingkey in this way.

brew install gnupg gnupg2 pinentry-mac
git config --global user.signingkey <YOUR_SIGNING_KEY>
git config --global commit.gpgsign true
git config --global gpg.program gpg

Put the following in gpg.conf file (edit file with nano ~/.gnupg/gpg.conf command):

no-tty

Put the following in gpg-agent.conf file (edit file with nano ~/.gnupg/gpg-agent.conf command):

pinentry-program /usr/local/bin/pinentry-mac

Update:

You might need to execute killall gpg-agent command after editing the configurations file, gpg.conf, according to the comments. As the self-explanatory command says, this command will terminate the GPG (Gnu Privacy Guard) agent.