Hi there, sourabh196.
Congratulations on your new iPhone 12 mini!
It appears that you're having a problem adding your Gmail account to your iPhone. You also mention that your Gmail account is also your Apple ID. WE understand how important communication through email is, and we're glad to help with this.
Sign in with your Apple ID - Apple Support -- If you're using your Gmail account as your Apple ID, go to Settings > [your name], then scroll to the bottom of the page and tap "Sign Out". Then restart your iPhone: Restart your iPhone - Apple Support. Once your phone is back to the Home Screen, go back to Settings > [your name] and sign back in. Then go to Settings > Mail > Accounts and see if the account has been added.
If this doesn’t solve the problem, please provide more detail. Also include the full text of any message you receive.
Let us know how that goes.
Take care.