Earlier quoted context omitted.
Number of people using Gmail is basically 100%
Your implausible estimate got me curious; so I checked[1,2] the first couple of result from Google on email marketshare. GMail is 27.7%. Apple has double the share, at 54%, you should be worried about Apple instead. 1. https://www.litmus.com/email-client-market-share 2. https://www.oberlo.com/statistics/most-used-email-clients
People add their Gmail account to their iPhone and get Apple Mail.
Virtually all email goes to Gmail between personal accounts and GSuite stuff.