To import MBOX to Gmail, you need to use an email client like Mozilla Thunderbird as an intermediary. First, install Thunderbird and the ImportExportTools NG add-on. Import the MBOX file into Thunderbird using the add-on's import function. Then, set up your Gmail account in Thunderbird by adding a new account and entering your Gmail credentials, ensuring IMAP is enabled in Gmail settings. Finally, drag and drop the emails from the imported MBOX file to your Gmail inbox or desired folder within Thunderbird, allowing the emails to sync and appear in your Gmail account.