While I do not work with Google Workspace directly, my org does have many, many applications and frankly it just very much depends on the app that you are integrating with and your specific needs.

For us, and in many cases, the Okta username and the user email is the same for each user, so naturally we have fewer issues than we might otherwise have. In your case, if the email is different, and that is the primary way you want users to login into and be provisioned into Google Workspaces, then certainly you should be good to change this to "email".

Also, sometimes we have situations in which the app has been in use for sometime and there are plenty of users already provisioned. If they were created with email, it makes sense to have this value match so its easier to do the mapping after integration. In other cases, I have seen that some apps require more customized values, such as Employee Number or something similar.

You can even use the Okta expression Language if you wanna get more customized, and the below is a great article that gives some examples (and shows just how many combinations and opportunities there are!):

How to Update Application Username Using an Expression Language

Please let me know if that helps! If you have any questions don't hesitate to ask away. Thanks!