As git bash uses it's default gpg program which is located by default at C:\Program Files\Git\usr\bin\gpg.exe and keybase uses the gpg program of windows which is installed by default at C:\Program Files (x86)\GnuPG\bin\gpg.exe so it's obvious to get the following error while performing commit in git bash -:

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

So it would be very helpful if you add this fix (to run the following command in git bash) in the documentation for the above error

git config --global gpg.program "C:\Program Files (x86)\GnuPG\bin\gpg.exe"

PS-: there is one fix like this for Git UIs but that takes the git bash default gpg program in windows