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

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.

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 recent systems 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 run a signed commit, you can verify it signed with:

git log --show-signature -1

which will include gpg info for the last commit.