https://gmail.googleblog.com/2013/12/download-copy-of-your-gmail-and-google.html
open/browse mbox file with freeware https://www.bitrecover.com/free/mbox-viewer/ (super extra speed) https://sourceforge.net/projects/mbox-viewer/
use thunderbird addon to import it (attachments supported) (ref) Paolo.ImportExportTools supports Thunderbird v14.0 - v60.x
2020 update
tested&working Thunderbird v68.5 with ImportExportTools NG (attachments supported) supports Thunderbird v60.0 - v69.x
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If you still want to use Outlook (convert mbox to pst), use the paid MBOX Converter Wizard.
If you want to do it manual (export all emails to separate .eml files, attachments supported), use got-your-back by jay0lee. Supports restoration by the eml or mbox.
by my tests, the best option is with Thunderbird (no credentials, export from gmail a file, import it), jay0lee solution is long standing.
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Restore emails online
Mozilla Thunderbird, is able to open .mbox files with an addon. After you’ll need to add the G Suite/Gmail account as IMAP and start Copy/Paste messages from the archived folder to the active account. src
Simply import the MBOX file to a folder contained in your Gmail account and your data will be effectively moved/migrated! src
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