I’ve had this issue on iOS 13 as well, but I think it’s an issue with the Apple Mail client rather than gmail. If I check gmail using the gmail iOS app, another client, or, of course, the web site on a computer browser, I’ll find mail in my gmail inbox that is not showing in the inbox of Apple’s mail client (or even in All Mail when I check that folder in the Mail app.) Obviously I’m not using the stock mail app as my primary mail client for my gmail accounts.