Gmail is still functional without paying anything. You don’t hit a paywall immediately after downloading. The app provides immediate functionality to users. If Google decided to force subscription-only, that’s when Apple would be saying you need to either open it up, or use our in-app subscription service. Google’s upsell is not a requirement to use Gmail’s basic functionality.
The app must provide immediately functionality after downloading. Guess how much functionality the Hey app I downloaded is providing to me right now? Zero. Because I need an “invite-only” code.