I certainly don't wish to be unfair to The Brain team. I have benefited from their enthusiasm and their prompt support of this Forum and of my personal emails sent on to them. I wish them success in their rewrite. Nevertheless I think it is quite reasonable to expect that sync of events with Google Calendar is part of Brain 9 Pro both in the licensed version and the continuing subscription version. In expectation of this feature I changed from a One Drive Calendar to Google Calendar a couple of months ago.
Then look at 'Top 10 Reasons to Upgrade to TheBrain Pro'.
This contains the sentence 'Also includes reminders and sync of events with Google Calendar'
I think it is quite rational to conclude that the 'sync' of Google Calendar to is something different to the 'sync' of individual Brains in the Cloud which is the topic of a tutorial.
When I was eventually able to find a way of purchasing a licensed version I was told to buy what seemed to be the Combo version and told not to renew for a second year. In doing this the features of Brain 9 would still be available as a desktop version which, as I say, could be reasonably assumed to include Google Calendar but could not be expected to include anything associated with the Cloud.