For all steps I'll assume user@example.com is your Workspaces user and user@gmail.com is your GMail user.
Do this first as Legacy Voice is going away at some point and I'm not sure it'll keep working when it does.
- Log in to both accounts in a single browser session
- Visit https://voice.google.com/ as user@gmail.com
- Accept the terms of service but DO NOT create a Google voice number
- Visit https://www.google.com/voice/b/0#phones as user@example.com
- Click 'Transfer' next to your number
- You should be eligible to transfer to user@gmail.com assuming:
- You don't have a Voice number on user@gmail.com
- You have accepted the ToS as user@gmail.com
Notes:
- No content will transfer, only the number itself
Allowing sending as user@example.com from user@gmail.com
Note: 2 Factor Auth must be enabled on user@gmail.com
- Visit https://myaccount.google.com/apppasswords as user@gmail.com
- Choose 'Mail' for app
- Choose Other under device and enter "smtp for user@example.com"
- Copy the password somewhere temporary (clipboard is fine)
- Visit https://mail.google.com/mail/#settings/accounts as user@gmail.com
- Under "Send mail as" click "Add another email address"
- Enter your user@example.com address details
- Make sure "Treat as an alias" is checked, click "Next Step"
- Enter
smtp.gmail.com
as the SMTP Server, port should be587
- Username is your gmail account, user@gmail.com
- Password is the app password we just copied
- Make sure TLS is selected, click Add Account
A confirmation email will be sent to user@example.com. Once confirmed you're able to choose that email address as a 'From' when sending.
You can reuse the same app password to set up as many email aliases as you need.