Please try the following fix:
1. Click on the Apple website below:
2. Sign in your Apple ID account.
3. Click on Edit on the right of the section Security.
4. Click on the x icon on the right of your gmail email address, then click on option Remove.
5. Click on Done on the right.
6. Next, click on Edit on the right of the section Account.
7. Click on the x icon on the right of your gmail email address, if shown, then click on option Delete.
8. Click on option Change Email Address...
9. Then enter your gmail email address.
10. Then click on Continue and continue following the instructions there.
When you are available, please reply at the box below to continue. Thank you.