If the calendar above doesn't show, you get an error message, or it wants you to log into Google and you don't want to:
Open an 'incognito window' in your browser and try again.
Be careful with this. Importing transits into your default calendar can create an overloaded calendar.
1) Login to Gmail or Google.
2) Create a fresh, empty, new calendar in your Calendar app.
Then import the calendar ical/ics formatted file into that new empty calendar.
If you skip this step, the calendar will import into your default calendar.
To undo that - you will have to delete your default calendar.
A new, separate calendar allows you control of when and where you see it, and lets you delete it easily if needed.
You can add the calendar by choosing Import, using this calendar ID:
37a4fc0f95d7f49fb257d9c5675011893d7d97fc1f5a48cdf5e5ed977b749bc0@group.calendar.google.com