Last update: 24 January 2019, 8:00pm
The current version is 2.1.34.447.
You shouldn't need to download this update manually, but in case you do, here is a link to the Latest Update Installer. This link may take some time to open -- Google wants to scan it for viruses before each download. After you download the file, close The Gradebook and run the installer. Once the update is complete, you may delete the downloaded file.
Here are the latest release notes: Release Notes