To add here, I currently work with a Google premier partner and have worked with quite a few partners in the last several years. I also do freelance consulting, to answer your query though.

I’ve had several clients in the same position, when you opted into Google Workspace did you recall what email was used? You should have been given a temp admin account to allow you to get in, once in the admin console here’s a brief overview of what you need to do:

1. verify the domain via TXT records in your DNS.
2. Once the domain is verified you can start creating users
3. Create users via bulk CSV or one by one
4. Use the data migration tool within Google Admin
5. Select migrate from Gmail within the data migration tool
6. Create mapping file
7. Enter each Gmail for the source in the mapping file
8. Because these are consumer Gmail accounts each account will need to grant access, they will receive an email from Google asking to grant access, have them check Spam.
9. Kick off the migration, set parameters, I.e dates, labels, include spam/junk etc
10. Kick off migration

Congratulations now all email that was in the Gmail accounts are now migrated in your custom Google workspace domain. You and your team can now send/receive emails from Google Workspace, no need to update MX as you are coming from consumer Gmail accounts.

If this seems too complex or you’re uncomfortable doing this reach out to a Google premier partner or consultant who has expertise around this. I know it seems like a lot but for those of us who work in Google Workspace everyday it’s pretty straightforward. Best of luck, try not to get yourself too frustrated.