Unlink my Gmail Address in my Apple ID Account

Good day!

My iPhone was stolen and the thief changed my Apple ID into his personal Gmail address. Now, worried about changing my other details, I removed the account of the stolen iPhone since it is connected to my other iPhone. In order to remove his email address, I have to add another email address, in this case, I used my personal Gmail address. Before the stealing incident, I already used my iCloud as my Apple ID which I really like.

My question now is, how to remove my Gmail address as my Apple ID account? I just want to make my iCloud alone as my Apple ID but now I can no longer delete my Gmail address.

Thank you.