The purpose was to transfer both Gmail contacts and Gmail emails. I didn't find a proper magic all-in-one solution so I used the following workaround.

Contacts

I had to use two different solutions to do so. I'll start by the easiest one: The contacts.

• You have to export them as "CSV Google", pretty straightforward:

1. Go to your contacts: https://contacts.google.com/preview/all
2. Click on "More" then "Export", if you're using the newest version of Gmail, it will ask you to go to the oldest, ex: https://www.google.com/contacts/u/0/?cplus=0#contacts
3. Click on "More" then "Export" and select what contacts you want to export and the format (CSV Google in our case)

• To import them:

1. Go to your other Gmail account and do the same steps, except you must select "Import" this time. The importation is pretty fast.

Pretty simple and quite fast.

Emails

The emails are quite more touchy. I tried to export them as ZIP using the Google Dashboard and it worked, but now way to import them that way. (so far, I haven't found one!)

I used Hiver for Gmail, a browser extension that brings several tools to Gmail. It's a free (14 fremium days, then ask for payment, but can still be used for free with limited features)

Note: I am not related to Hiver in any way, just a tool I discovered a year ago, quite useful IMO.

Website: https://hiverhq.com

Chrome extension link: https://chrome.google.com/webstore/detail/shared-mailbox-shared-gma/fcinnggknmdfkilogcndkgpojpfojeem/related?utm_source=chrome-app-launcher-info-dialog

Anyway, install the extension depending on your browser and follow those steps:

1. Once Hiver is installed as an extension, go to your Gmail account. You should see a link with Hiver on the top header of Gmail. (On the screenshot, it should be on the top-right, where I have 3 green icons (because I'm already authenticated))

2. Log in (it uses the current Gmail account to create a Hiver account and log you in automatically, no password)

3. Repeat the same thing with the other Gmail account.

4. Now you should have Hiver installed on both your Gmail accounts, and logged in.

5. Take any of your account and create a Shared label. (Please follow Hiver official documentation https://hiverhq.com/tour), share the label with your two gmail accounts. I called my label "Duplicated", name it as you like it.

6. Refresh both your gmail tabs. (you never know)

7. Try it around, apply the "Duplicated" label on one email and see if it gets duplicated in the other account (it won't be duplicated in the "Inbox", but in the "Duplicated inbox"). Duplication may take up to 1 mn, usually faster.

8. If everything is smooth, then select all the emails you want to duplicate and add them the "Duplicated" label all at once. (In my case, I selected all email (50), then Gmail proposed me to select all emails from the inbox and I clicked on the link, it selected more than 1100 emails, and I applied the label)

9. See the magic in action. Emails start to get duplicated on your other Gmail account. It takes time. (I think about 45mn for 300 emails, in my case. So probably a few hours to get everything duplicated) It's fast enough IMO.

10. You can close all windows, shutdown the computer, whatever, the process is done server side and you can forget about it and see the result in a few hours. I haven't tried to mess with it by removing "Duplicated" label or anything, I don't know how it may behave if you do so :)

And here we are. If you enjoyed the software you can buy a premium account. Hiver has much more features like email reminders, scheduled emails, email templates which I find pretty useful.