Mail app login

If you already used your Apple ID to setup the device, then -- Set up iCloud Mail on your devices - Apple Support.

"Set up an iOS or iPadOS device

• On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud, then turn on Mail."

You have to download mail app first from AppStore

does your gmail and iCloud enail are same

type your email(google)to sign in