Found earlier versions of the plugin here
https://wordpress.org/plugins/google-calendar-events/advanced/
Scroll down to the bottom, you will see a dropdown where you can select the version you need to download.
I reverted the plugin to the earlier version and my site is back to working.
I have the same error after upgrading. It looks to me the developer is using code which can only run in php7.1. Is there any way to download the earlier version so that we can revert? Thanks.
Looks like the behaviour is correct. What is your problem then?
