The config for the Simple frame confg asks for a relative path. Given that how would one enter a Google calendar path?

Have you read the plugin description ;)?

Depending on browser origin policy restrictions of frontend users it could be possible to embed only pages inside gpEasy root!

So it's not recommended to use it with an url outside gpEasy root. you can try it, it will maybe work on your browser but there's no guarantee that's the same on other browsers like the current one with higher security levels.

So the plugin asks for a relative path like /forum (will be http://www.your-domain.tld/forum).

Can a solution be to use an HTML wrapper? The page URL which lies outside of gpEasy can be held within a wrapper. A wrapper snippet would need to be made for gpEasy.