Apple released their iOS 14 software update last week and this allows you to change the default email and browser apps on your iPhone and iPad.
Google has now released a software update for their Gmail app on iOS and the update now supports using Gmail as the default mail client on iOS 14.
If you want to swap from Apple’s mail client to Gmail as your default mail app on your iPhone or iPad, you can do this by going to Settings > Gmail > Default Mail App and selecting Gmail.
The latest version of Google’s Gmail mail application is now available to download for Apple’s iOS 14 devices from the app store.
Source MacRumors