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.
Now, enter the App-generated password for MacUncle and use the filter option for selective conversion of MBOX to Gmail.
Then, press Export button to migrate MBOX to Gmail account.