The Apple ID created with gmail account say example1@gmail.com was permanently deleted from the gmail server , and if the account was set up with secondary authentication , the administrator can’t sign with iCloud account.
Sign in with rescue mail address in gmail account ( enter the password ) , click on personal info .
Click on edit , and try to add a new trusted number .
And can't further move , the bell icon will shake and stop and if you click on send the code that will go to the old trusted number that is inactive .
Click on more ways to verify , it again prompted to enter rescue mail address and password , Scroll down and click on account recovery icon , then add a new trusted number and click on next , the trusted number will add , the trick was using the rescue mail address that was attached to the primary mail address .
See the links https://support.google.com/accounts/answer/7682439?hl=en
https://support.google.com/accounts/answer/7299973
Then their device can be removed Remove a device in Find My on Mac - Apple Support