Hi There!
We are currently in the process of reviewing your linked documentation article, since it doesn't account for the initial setup wizard some distributions will run on the postfix package.
Here you can find a link to the report:
Now, for gmail accounts, you usually need to set up an App Password, since gmail forces MFA on regular logins these days, and you cannot use MFA with postfix:
Once you have created the app password, you can go back to step 3 of the guide.
Let me know if this helped. If you continue to face issues, you can send me a copy of the
/etc/postfix/main.cf file for review.
Regards,
Federico