GAuth is written in Python and QML (via the PySide bindings) and uses the Harmattan Qt Quick Components library. It can be downloaded from the Nokia Store or GitHub - where both source code and installable binaries can be found.
MeeScan application, also available from the Nokia Store. (Note that the current version of MeeScan sees the QR code as text rather than a URL, you need to copy and paste it into Web to activate the account in GAuth). Other apps are compatible too, including CodeCam. Alternatively the account can be manually configured using the Settings application (tap on Applications then GAuth - this may only appear after a reboot of your phone once installed the first time).
For more information on GAuth please read the README file or the help file (a copy of which is provided in the application from version 1.3 onwards).
Update 19/05/2012: GAuth version 1.3 is now available in the Nokia Store.
Update 29/08/2012: GAuth version 1.4 now on GitHub. New version contains Dropbox compatibility, multiple account support and an optional inverted theme. See this post.
Update 02/09/2012: GAuth version 1.5 now on GitHub. Bug fix for new users (1.4 wouldn't have started).
Update 04/09/2012: GAuth version 1.6 now on GitHub. Regression fix for Google users caused by me adding Dropbox support. Submitted to Nokia Store QA.
Update 06/09/2012: GAuth version 1.6 now on Nokia Store.
Feel free to post issues on the issue tracker at the GitHub repository or as comments to this page.