Apple - Is there any way to set Gmail app as the default mail client in iPhone and iPad?

To give a very short answer: No, this is not possible.

This is not possible on iOS 13 and lower. (Or iPadOS 13 or lower)

In order to utilize email functionalities from within apps, you will have to enable the Apple Mail app. I recommend turning off all notifications for Mail if you want to use the Gmail app as your primary iPhone mail client.

It works if you are using the Gmail app and Chrome browser (i.e. when clicking on the mail icon in a NY Times story you are browsing in Chrome), but I don't think you can enable it by default in other apps.