Found that the embed only works if you take the plain URL from the “Embed This Calendar” box in Gcal settings
If you click on “Customize etc.” and paste the long URL with all the parameters separated with & then the important bit (src=6hnjp743blahblahblah) doesn’t make it through.
I am not a coder so I don’t have the first clue about how to a)track down this issue in the source or b)submit a pull request to fix it, but hopefully this helps someone else with the same issue!