You can easily import MBOX files to Gmail using MacUncle MBOX to Gmail Converter. The tool supports Mac OS Machines.
Start MacUncle MBOX to Gmail Converter on your Mac Computer.
Select MBOX file using Add File(s) or Add Folder(s).
Choose the desired files from the software panel.
Click on the Gmail in Select Saving Option and then enter Gmail Credentials and IMAP Port and IMAP Host.
Then, press Export button to migrate Mailboxes to Gmail account.