This worked for me on Windows 10 (Note that I use the absolute path to gpg.exe):
git config --global gpg.program "C:\Program Files (x86)\GnuPG\bin\gpg.exe"
This was the error I got prior to the fix:
gpg: skipped "3E81C*******": secret key not available
gpg: signing failed: secret key not available
error: gpg failed to sign the data
fatal: failed to write commit object