Is there a Google Authenticator desktop client?

Install oath-toolkit via brew, generate your keys with:

oathtool --totp -b <your_secret>

To get your secret, use the 'change phone' option in Google. You can get your key by clicking the 'manually enter the key' link.

There is a Self Hostable Web App at https://github.com/gbraad/html5-google-authenticator This is not secure on an internet facing machine, but you can load this on a completely offline Machine and you'd be safe.

Yes. You can use WinAuth. Works fine for me.