It is not me who is using gCal, but an associate who invites me to events using their gCal. I also have an account (who doesn't?), which I prefer not to use. I have reasons for wanting my events all in my iCloud calendar, for example it is shared with my partner so they can plan more easily.

No, almost the opposite. I am looking for a way to accept invites from a Google calendar user to my gmail address, while adding them to my iCloud calendar. I do not want to be forced to have some of my events in a Google calendar, but I still want to accept them (and ideally get nice updates when the event changes).