How do I change my Apple Account to Gmail, and then delete the iCloud email?

see this on how to change your apple accounts primary email

Change your Apple Account primary email address on your iPhone or iPad

1. Go to Settings, then tap [your name].
2. Tap Sign-In & Security.
3. Tap your primary email address to make changes.
4. To remove this email address from your account, tap Remove from Account. If you want to keep the current email address associated with your Apple Account but change to a different primary email address, turn off Primary Email.
5. You’re prompted to choose another primary email address. You can either choose one of your other email addresses associated with your account as the new primary, or add a new primary email address.
6. Follow the onscreen steps to enter and verify your new primary email address. Learn what to do if you didn’t get the email.