This is an electron app wrapping https://calendar.google.com. It provides nothing on top of Google Calendar other than putting it in an independent window.
Check out the 1.0.1 Release to download a DMG containing a pre-built app.
To build the app use the Makefile.
make
If you're lucky, you'll then have the file target/google-calendar-darwin-x64/google-calendar.app
.