I used Thunderbird and a plugin that can import MBOX files. Works like a charm, and as we speak I'm uploading folder by folder to Gmail using IMAP from Thunderbird.
Make sure you export the folders one by one (except all the Sent folders, which I exported as one large file).
Then, follow the instruction at https://backupify.zendesk.com/entries/20644...-Gmail-account-
Good luck!