Adding iCloud email address to GMAIL.
Want to add iCloud email address to GMAIL but it is rejected with a dumb note to go back to Apple for special password?
Question marked as
Top-ranking reply
To sign into iCloud in a 3rd party (non-Apple) app, you need to turn on two-factor authentication if it is not already on and you need to use an app-specific password rather than your usual Apple password. See the following for more information:
Two-factor authentication for Apple ID - Apple Support
and