unable to change my Apple Id email address
I'm trying to apply my gmail address as my Apple ID, but the procedure keeps rejecting the email that I try to set.
This gmail email was set previously as my notification email, so I changed the notification email but I still fail.
Any idea how can I set my gmail as my primary Apple ID? I tried both on my iPhone and on the desktop application. Both fail.
Thanks!
iPhone 6, iOS 11.2.1