Can't verify my email address
I just upgraded to a paid iCloud account. My apple ID is my gmail email address. It works whenever I want to download an app, etc. I can't log into my iCloud account to manage it, however, because when I'm asked to verify my email address and a verification code is sent to it, the iCloud website won't accept the code because "This email address is not available. Choose a different address." This doesn't make sense. Help?
iPad Pro, iPadOS 13